Missing block texture

This is probably because they start removing this item with the data value, because it was used in earlier versions when there was no "wooden_slab", but then they added it (also to fix the bug, that the slab can be destroyed quicker with a pickaxe) and now you don't really need it anymore I guess 😉

I notice it with the bookshelf block.

It's because you're getting an item that doesn't exist. Works as Intended.
See MC-58445

well actually it does exist, and more of a duplictae from MC-58287 as I mentioned this block there as well

You're not intended to be able to acquire these items any more, though they haven't quite dealt with the old fake wood stone slabs yet, as they still appear correctly in the world and can be picked up using a pickaxe, but don't render properly as an item.