Minecraft version 1.21.1
Fabric Version 0.16.7
Blockus 2.9.5
Extended 3.0.2

I tested just now and could not reproduce the issue. Could you use version 3.0.4 of the Extended Block Shapes mod?

Sure, where can I find 3.0.4?

I have a massive pack so its probably another mod conflicting. Let me do some more troubleshooting.

Ok I found the conflict.
My bad, sorry to waste your time.

It was Arcle's Medieval Containers Resource Pack.