1.20.1 Double Slab from different material still exist as 'air' when broken down

[Horf]

To replicate I put a wooden slab on cobblestone and on top of the wooden slab a cobblestone slab.

I used a pickaxe to remove the cobblestone slab, both slabs seemed to vanish, but instead they were still there as thin air. Had to use /setblock command to get rid of the "ghost" slabs. You can't see the salbs and there are no dropped slab items in the world, but they still have collision.

So it's not possible to disasamble slabs of different material - this doesn't happen if both slabs are from the same material e.g. both are cobblestone.

Edit:
It also seems that I can't place a vertical slab (of cobblestone) while I'm sneaking.

[CJMinecraft01]

Could you attach a video showing this?
I've tried to recreate this and have been unable to - it's possible that you have another mod installed that is conflicting with this one.

[Horf]

I just tested it with the latest release from 2 days ago (for Forge 1.20.1) and it doesn't happen anymore, so whatever the cause was, it's fixed now.

Still it seems I still can't place vertical slabs or I'm doing something completely wrong...

When I place a vertical slab (how am I supposed to craft them? For the moment I just cheated one) it looks like it's placed horizontally, but the collision is placed vertically (as seen in the following screenshot):
https://ibb.co/MxQkpVTQ

[CJMinecraft01]

I'm glad to hear that it's fixed! To craft a vertical slab, place a regular slab in a crafting bench and it's a one to one conversion.