Steps to reproduce:

• turn anti-aliasing all the way off
• turn anti-aliasing all the way on
• open "pause" menu
• open another window (windows key, click on something in the taskbar)
• re-open minecraft (DON'T CLOSE MINECRAFT COMPLETLY)
• Observe

Expected results:
smooth distant textures (file 2021-05-12 (1))

Observed results:

weird lines and flashes (files 2021-05-12 & 2021-05-12 (2))

Notes:

1. I first noticed this with the Addition of RenderDragon.
2. I use an AMD Radeon RX 5600 XT.
3. There are similar bug reports, but none explicitly described my problem.
4. the AMD software doesn't seem to override the in-game options.