Creating double slabs with 2 slabs will delete the top slab/will not allow top slab to be placed on bottom slab

Top slab gets deleted when placed on bottom slab

to reproduce
1. place bottom slab first
2. place top slab

The top slab will be placed on the bottom slab but it will immendiately disappear. But if you place a top slab near a full block (needs to be on the top of the full block), the top slab will not get deleted.