Minecraft: 1.21.4
MoreCulling: v1.2.2
Classic 3D: v2.0 Alpha 9

The blockstate culling option is over aggressive with mob spawners when using the resourcepack Classic 3D. As the author of the resourcepack, I don't see a way to fix this on my end. Most likely is to do with how mob spawners don't have normal culling, but the model is correctly culled in vanilla.

This does indeed look like an issue on our side. We will look into it.

should be fixed in 1.2.6, sorry for taking so long to fix it

Tested, and yes it has been fixed.