The bug
Armor stand items with Small:1b set in their EntityTag cannot be in 1 block high spaces.
How to reproduce
Give yourself an armor stand item
/give @s armor_stand{EntityTag:{Small:1b}}/give @s armor_stand{EntityTag:{Small:1b}}Try to place it in a 1 block high space
I change the report a little bit, I hope you are fine with that.