I'm using fabric 1.18.2 with a mod list with OkZoomer, Optifabric and the most recent version of Optifine. I´ve tested my mod list without the mod and it works perfectly.

okzoomer-5.0.0-beta.5+1.18.2 messes specifically with optifine's glass and glass pane connected glass textures. Optifine's grass blocks connected texture, for example, works as intended, although I never use this option. Only turned it on for testing.

Due to OptiFine not being particularly mod-friendly, it and OptiFabric aren't supported. However, I believe it's the same underlying issue as #57. Could you check if this still happens on 5.0.0-beta.6?

Unfortunately, glass textures remain separate with the most recent beta.

So while I've got no idea why this happens, I found a temporary fix: Simply by extracting Optifine's resource pack and enabling it manually, it worked.
Optifine_fix.zip

This works perfectly, thanks a lot!

Stale issue