Still happens on mGBA-0.8.2-win64 but less regularly.
See the savestates attached.
mgba-1711.zip

With 0.8.2 some really quick screen flickering started to appear.
I've recorded it because I cannot savestate it. Happens bettwen 0:07 - 0:08, 0:11 - 0:12
Video is in the attachement, too.

Still happens on mGBA-0.8.3-win64.
I additionally tested it on different scalings to rule out upscaling as an issue. The issue affects both the original scaling/aspect ratio as well as increased scalings like 2x or 4x.

I had trouble reproducing this when I tried. I'll try again to get it fixed in 0.9 (assuming you've tried it in a dev build already?)

Yes, tested it on previous dev builds and just now on build 6385-453791d.
Glitches still occur. Unfortunately I couldn't use my save states on the dev build. Loading it fails with:
Jumped to invalid address: E3A03300

Memory address displayed is consistent on every attempt to load it.

Savestates can be flaky like that. I'll look into it, and if possible I'd appreciate making a new savestate. Otherwise I'll try grabbing the savegame out of the state and using that (assuming there is one in there; there isn't always).

Gimme a sec, I'll attach new save states plus a save file for the dev build.

There you go, save file and save state showing the permanent glitch.
Flickering can still be reproduced by talking to NPCs and skipping through their dialog.
mgba-1711-dev.zip

I can't reproduce this on Linux/Intel GPU, but I CAN reproduce this on Windows/Nvidia GPU. It may be a driver bug. Will continue to investigate.

I think I figured out why I can't reproduce it; it's not an OpenGL bug, it's a bug that doesn't happen if using the official BIOS. This appears to be another manifestation of the same bug in #1823.

If there's any more testing I can do or just driver information I can provide feel free to let me know.
Will keep testing on newer dev build every now and then.