Got a bit sad when they immediately closed my ticket, glad to see you created a new ticket confirming with even more tests I hadn't thought about.
I'll re-test once the hitbox is fixed to make sure this isn't a separate and newly introduced problem with diagonal hitbox obstruction. I agree that it could be a duplicate, but I don't see how the height of the stair hitbox should make you hit the lapisblock in the pictures. If it was above the stairs, fine, but "inside a wall"?
Please notice how this post also refers to snow and slabs. Are you saying snow/slabs are stairs? If this somehow is because of the same problem, the hitbox problem needs to be expanded beyond "stairs", because as Zuriki reports, this does not only happen with stairs.
This is NOT a dupe of MC-61245.
If anything, MC-61245 could be related to this post, but only partially.