No idea what's happening here. Sony 7R4, iso100, raw file. File itself is perfect in Library but shows extreme pixelation when any edits are made in Develop. Previews have been reset, Lightroom reinstalled, all software and os are up to date.
Sent the files to another computer and they look perfect. So I know this is an issue with either Lightroom or something gpu related.
First, try disabling GPU in the preferences (Performance). Any better? If not, recalibrate and build a new ICC display profile; the old one might be corrupted. If you are using software/hardware for this task, be sure the software is set to build a matrix, not LUT profile, Version 2, not Version 4 profile.
Be sure to view in Develop at 1:1 for the most accurate previews too; zooming out can in some cases produce a moiré due to subsampling of data, but in your screen capture, this really looks like a GPU issue.
Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"