I have started working with PSE 2020 on a brand new Mac Mini (all up to date), and while working on a bunch of jpg photos from iPhones I noticed that in some zoom levels random vertical and diagonal lines appeared in the photo I was working on. Only visible in PSE, not in other programs at that point. At first I thought that it is a display issue, but in one case (setting up a collage) it happened that these lines got saved with the photo, so that they are now visible at any zoom level, and other programs show them as well.
I verified the original photos, before PSE touched them, didn't have these lines
I tried different preferences and jpeg saving settings, made no difference
Preference settings control how Photoshop Elements Editor displays images, cursors, and transparencies, saves files, uses plug‑ins and scratch disks, and so on. If the application exhibits unexpected behavior, the preferences file could be damaged. You can restore all preferences to their defaults.
Note: Deleting the preferences file is an action that cannot be undone.
Do one of the following:
Press and hold Alt+Control+Shift (Mac: Option+Command+Shift) immediately after Photoshop Elements begins launching. Click Yes to delete the Adobe Photoshop Elements settings file.
Go to Edit > Preferences (Mac: Photoshop Elements > Preferences > General), click the Reset Preferences on next launch button, and then click OK. When you restart Adobe Photoshop Elements, all preferences are reset to default settings.
A new preferences file is created the next time you start Photoshop Elements Editor. For information on a specific preference option, search for the preference name in Help.