artifacts in timeline but NOT browser

So I'm working on a 1080 timeline and 4k ProRes files.
Files left in place (external hd), not imported to library. Mac Pro is loaded so it's good.
No problem so far. 6 projects in after I make one cut I notice the same artifacts on the video in the timeline. Same place. I check the file in the browser and it's clean. I replace it on the timeline. Still there. Make duplicate project. Still there. Trash the preferences, still there.
So I start searching forums including this one but I only find discussions about codecs, etc.
It's not until I select the timeline clip and futz with the Spatial Conform - which was set to FIT (since it's a 4k clip into the 1080 timeline). Fit created artifacts (out of the blue in this library on just this clip). Set it to FILL and artifacts gone. Set it to NONE and gone too.

SOLUTION!

So I wanted to leave this problem + solution here for anyone else having this issue to find.

The first thing to try in situations like this is to delete all the render files. Glad you figured it out.

YESSSSS!
So much MORE logical. It kept showing up bc Final Cut was just reusing the same render files... In my head I just kep hearing the common "trash the prefs, trash the prefs"... Next time my goto will be DELETE THE RENDERS first.

thx!

And make sure in the Preferences you have Background Rendering turned off.

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