if 2 non-wooden slabs are placed on top off eachother the texture changes to the smooth-double stone slab texture

*What I expected to happen was the 2 slabs would create a solid block

*What actually happened was the texture changed to the smooth-double stone slab texture

Steps to Reproduce:
1. place 2 non wooden slabs on top off eachother, in the way they would normally create a solid block