With banners, when the actual block is off-screen, the entire model is culled. This means that banners can be culled while still partially on-screen, as part of the banner model extends beyond the actual banner block. In short, invisible banners. This happens with both standing and hanging banners.

The bottom block is partially on-screen:

Now that it's off-screen, the whole banner disappears:

The top block is partially on-screen:

Now the whole thing's gone:

All screenshots were, of course, taken from the same place at the same time, just with a slightly different viewing angle.

I'm on 1.21.10, using EntityCulling 1.9.0. I've tested with and without shaders, and have ruled out any other culling or block entity-related mods.

Ah this is a 1.21.10 fabric issue only (I assume), I'll have to give banners a bigger cull box.