When the Fabulous! video settings are chosen, the rendering becomes very glitchy and unusable. This could be related to MC-188669, but I have decided to open a new bug report because that one is marked as resolved, and this one has far worse effects.
I have disabled mods and the bug still occurs. Notice that in the big window, the block in the centre is regular clear glass, while the others around it are black stained glass that add another trasparent layer. Several other bug reports talk about a warning when switching graphic settings to fabulous, but I haven't met any with my system.
I have attached screenshots of the same spot, with rainy and clear weather, on fancy and fabulous video settings for comparison.
This bug has also been reported to Mesa, see #4026. There I have also uploaded a small video demonstrating the issue (it's 25MiB).
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You might want to report this bug to mesa too: https://gitlab.freedesktop.org/mesa/mesa
I'm not sure if the bug is on Minecraft's end or on Mesa's end. I do know that there have been incompatibilities between Minecraft's Fabulous graphics mode and mesa drivers before (see MC-187411).

Hi, thanks for reaching out! Good idea, I have now reported this on Mesa's end too, see #4026.
Several other bug reports talk about a warning when switching graphic settings to fabulous, but I haven't met any with my system.
Just to clarify, that warning has since been removed since the incompatibilities I talked about were fixed on Minecraft's end (it was a MC bug that was exposed only by Mesa drivers).

Is this still an issue with the latest mesa version?

I wish that I could test this, but unfortunately a few weeks ago the Radeon GPU in my laptop died, so there is nothing that I can do. The fact that my GPU is now faulty has nothing to do with the bug, as several people on #4026 have been able to reproduce it. I hope that someone else will be able to test this with the latest mesa.

@miwob, I have asked on the mesa bug-tracker, and the bug is still reproducible with mesa git updated from oibaf and MC 1.16.5.