when you place a stair or a slab on the 8th pixel down (top half of the block) the stair/slab will place on the bottom half of the block. This happens every time from my testing. You can also test with a slab by placing another slab on the very bottom pixel of a top slab.
Screenshots show where to place stair/slab to replicate the bug.
1. Not a web service issue.
2. Already reported, not bothering to search atm.