When zooming in and out while scrolling through this PDF file, I got this:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/0uggxmzaepzxc386w806.png
Slightly zooming out fixes the problem:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/787r25heha5hwcierpa8.png

Or:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/na39mwltbkunbc9c3790.png

Same problem with Sepia:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/gwhhohbmy2zsofu2fu4u.png
instead of:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/agt5zbelzwxevsu061jl.png

I also see the problem with the Original them, but less often. It may be just because it is faster to load:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/ftg6mojsoq48713i0ayu.png
instead of:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/v2o26j7897re813z2upr.png

It seems to be triggered if I zoom in or out while content of the PDF file is loading.

I have tested the bug while testing this other problem:
https://forums.zotero.org/discussion/comment/496800/#Comment_496800
But I don't know if it is related.

Zotero 8.0-beta.3+8355a9540 (64-bit)
Windows 10