The bug

Spawning an armor stand with NBT in the lowest y layer of a structure block and saving it does not generate the armor stand, only if it is one layer above or higher.

How to reproduce

1. Mark an area using a structure block and select to include entities.
2. Spawn an armor stand with NBT inside this area, for example: /summon minecraft:armor_stand ~ ~ ~ {Marker:true}
3. Save the structure and then load the structure.

The armor stand with NBT will not be generated.

1. Spawn an armor stand with NBT inside this area above a block, for example: /summon minecraft:armor_stand ~ ~ ~ {Marker:true}
2. Save the structure and then load the structure.
   The armor stand with NBT will be generated.