FCPX 10.4.3 Insert a frame during editing

During the video editing, my video was inserted many frames everywhere, here is an example, at 16:27:47 and 16:27:49, it should be a continues video, but suddenly a frame (16:27:48) inserted into it, which make my video has a "twinkle" at that time, I need to manually delete all of them one by one.

Any one know the reason for it?

There is no video example attached to your post. At what point during your editing did this odd behavior occur, and specifically what editing commands were you using?

Hi, thank you for the help. I just added new pic for your reference.

There is no odd behavior, just cut, add effects and combine.

I see what you mean, that middle frame doesn't belong there. I'd suggest the first thing you do, place the playhead at that frame, zoom in (Cmd-+) a LOT, as much as you can, see if you accidentally have one stray frame stuck in there. Then you can simply delete it. You'll end up with a Through-Edit edit point, which you can then safely Delete.