Render Artifacts - Show up sporadically after bounce

I bounced a film from FCPX and set the codec to H.264 for youtube. When I watched on it on quicktime and on youtube the first time I would get a really obvious and harsh ring where a smooth vignette was supposed to be as well as some other mask artifacts. However the crazy thing is it only happened in full screen and when the playbar disappeared. If I moved the mouse and the playbar came back all the artifacts disappeared. What's even stranger is that this stopped happening after a while long enough that I felt ok to send to a few people and now a week later it shows up again. No one has mentioned seeing this so far. For sure it's something wrong with my computer however I have never seen this happen on any other video ever, mine or otherwise. My worry is that this will appear at random on some people's computer. Does anyone have any idea what the heck is going?
Thanks Bros and Bro-ettes

Please give the specs of your media and your project properties.

As Tom said, we need (1) The properties of the original media and (2) The properties of the encoded file. Play those in Quicktime Player and do Window>Show Movie Inspector, or just CMD+I. List all the items for both original media and the encoded output file.

Do a screen cap of the behavior and post that.

We also need the specifics of your Mac, including year, model and config, and what version of MacOS it is running.

Is this only happening during Youtube playback, not when playing the local file on your Mac?

If so when playing it on your Mac, right-click on the Youtube window and pick "Stats for Nerds", and state those items. Also try the playback in a different browser, e.g, FireFox. Do NOT use Chrome, a browser based on Chrome or have Chrome running when FCPX is running - it can disrupt your entire machine.

mac os: 10.12.6 (16G29)
processor: 2.3 GHz Intel Core i7
memory 6 GB 1600 MHz DDR3

from quicktime:
Formate: h.264, 1920x1080 aac, 48000 Hz, stereo L R
FPS: 23.98
Data Size: 694.7
Date rate: 11.92 Mbit/s
Current Size 1440 x 810

It happens on Quicktime as well. I may have very well had Chrome running while working on FCPX but I wasn't aware of that issue and none of these problems have shown up when I watching my project on FCPX. Maybe is disrupted the export? I tried bouncing it twice btw and this exact issue happened. When I took a screenshot it didn't appear in the screenshot and by the time I had got to grabbing a video capture it the issue stopped again like it did a week ago. Hope this is helpful please let me know if I've missed anything you need.
Thank You

I just had to totally uninstall Chrome because it was messing up Final Cut. The first step, totally uninstall Chrome and all of its support files. Move the Chrome app to the Trash. Move this whole folder to the trash:
~/Library/Application Support/Google/Chrome

If you have the problem sometimes, and sometimes not, then it isn't your FCP or the files you're exporting, it's some OS or hardware level issue.

Thank you for your help. Is there any chance that it will happen on other people's computer before I resolve the issue and rebounce it? Or is it safe to share this and it's just some issue on my computer? And lastly when you say it's in issue in the hardware or OS system is that something that could be resolved with an update or will this require a deep dive? This is the first and only time it's happened and it seems to be related to some very specific effects in this video.
Thank you again you've already provided so much help. I do really appreciate and little bit of information you might be able to offer.

You previously said "set the codec to H.264 for youtube", then later "it happens on Quicktime as well". Anytime you have an issue like this do not use the Youtube export, rather export to a local file, inspect it, then upload to Youtube in a separate step. That way you always have the local file to examine. Maybe you have already done that.

Disable background rendering in FCPX Preferences>Playback>Background Render (clear check box). Then delete all render files: File>Delete Generated Library Files>Delete Render Files>All.

Then select all clips in the timeline with CMD+A, then do a one-time render with CTRL+R. After the render completes, export using the below parameters and play the file using Quicktime Player. If it does not look OK, play it using the latest version of VLC. If it does look OK upload to Youtube then after it finishes transcoding on Youtube to 1080p, play it up there using Safari (not Chrome or any other browser).

Use this export: File>Share>Master File>Settings, Format: Computer, Video Codec: H.264 Faster Encode, Resolution: 1920x1080

It could also be a transient or incipient hardware problem so run Apple Diagnostics or Apple Hardware test and report the results: support.apple.com/en-us/HT202731

Hey all, thank you for providing help. I took all your suggestions and for now it seems that it has solved the issue (and the final export looks a bit better overall, so I'm gonna assume it solved my issue). Wasn't able to diagnose any OS or Hardware issues but it's an old computer and an old external running low on space so it would not surprise me if that had some effect. Thanks again for all your assistance. Was incredibly helpful.

Hey guys I'm back... so after a few days I looked at this video again on youtube on safari and the same thing showed up again. It doesn't show up when I screen capture so the best I could do was get a little bit of footage on my phone. but not sure if it really shows up or shows how severe it is in person. I might just go ahead and assume it's only going to happen on my computer sometimes but it's such an oddly specific artifact that I've never seen on other published work on this computer that I'm worried it has something to do with my export or my rendering process. Though I did everything that was suggested in this thread. Any other ideas?

Give us the link on YouTube so we can see what you're talking about.

i would but there's a high probability that it won't show up. So far no one I've shared it with has had the problem. My concern is just that eventually it will be a problem somewhere simply because this isn't a problem that I've experienced on my computer with any other video. I attached a video I'm not sure why it's not showing up. Could you confirm if you see the video or not? Sorry I'm new here and not sure why the attachment function isn't working.

I do not see a link to a video on YouTube.

Well, it will look slightly different on different computers, different monitors, different browsers. Same with television broadcast, I know things won't look exactly the same universally on every TV set in the world. So you grade for the best image quality, release it into the world, and move on to the next project.