Saw this now 2 times. An amor stand falling trought a slapfloor to the next full block.
Searched in the bug-tracker but no one seems to have this, and found only old, solved things about that bug.

the amor stands was crafting trough the crafting-table and set with right-clicking. (not trought a command) and a few days later one of them was falling trought a slab. The next was again a few days later. The chunk was'nt unload, because i was in the cellar from this house.

The attachment show the slab-floor and the affected amor-stands.