When creating a double slab, which is a version of minecraft:stone_slab (for example, stone slab, sandstone slab, cobblestone slab), then it will always show the texture for normal stone slabs. An interesting thing about it is that it will have the same texture on all six sides, so the line between two slabs isn't visible anymore. It is possible that some other slab types are affected as well.
Environment
superflat, creative (possibly other gamemodes and world types as well)
Duplicate of MC-53370