I've seen this issue in v19, and it's still present in v20. On Ultra HD timelines, when motion estimation is set to AI Speed Warp Faster, the footage appears pixelated. In all other modes, it looks fine.
Here's a zoomed-in screenshot using AI Speed Warp Faster:

- Pixelated.png (13.06 KiB) Viewed 273 times
And here’s the same frame using any other mode:

- Normal.png (8.58 KiB) Viewed 273 times
The test footage is 3840x2160 at 25fps, featuring a black solid circle on a white background. On a 1920x1080 timeline, everything looks normal—this only happens on a 3840x2160 timeline.
The pixelation doesn’t appear on every frame. You need to move the playhead to spot it, since not every frame is interpolated.