When sign buffering is turned on, the text doesn't render, but the glow layer seems to render randomly.

With sign buffering on

With sign buffering off

Which other graphics mods are installed

Better Biome Blend
Clear Skies
Continuity
Cull Leaves
EntityCulling
Extended Clouds
Iris Shaders
Make Bubbles Pop
Model Gap Fix
No More Purple
Phosphor
Raised
Reese's Sodium Options
Show Me Your Skin!
Sodium Extra
Sodium

and how are the signs loaded? (log in next to them/walking them into view distance?) Also singleplayer or multiplayer?

It appears to be both when logging in and when walking towards them. It happens on both single and multiplayer.

Further testing revealed its actually just the glow layer, and it also leaves random pixels of the text. This is on singleplayer, the first screenshots are on multiplayer.

Huh. Which other graphics mods are installed, and how are the signs loaded? (log in next to them/walking them into view distance?) Also singleplayer or multiplayer?

Hmm this is odd. So far have no one else reproducing this. And the glitched text looks like something messed up the render pipeline before the signs get rendered out. Once we figure out which exact mod/situation causes this, then I can try mitigating the issue.

This same thing is happening to me, except the sign text doesn't show at all, even when it's glowing
Still happens even with just Fabric API and Exordium

Assuming you are on 1.20, please use #92.

Actually gonna close this one, since there are no reports like this since then, and a lot has changed.