When an entity mounts a display entity, internally, the Y position remains the same as it was before mounting, and becomes vertically offset from the mounted display. Relates to MC-259816 and MC-259999.

To Reproduce:

1. /summon minecraft:block_display ~ ~ ~ {block_state:{Name:"stone"},Tags:[test]}
   

2. Move in all three axes so that you are no longer aligned to the display

3. /summon minecraft:armor_stand ~ ~ ~ {Tags:[test2],NoGravity:1b}
   

4. /ride @e[tag=test2,limit=1] mount @e[tag=test,limit=1]
   

5. /data get entity @e[tag=test2,limit=1] Pos
   

6. Notice the position saved in NBT is aligned to the block display on the X and Z axes but incorrect for the Y axis only

7. /execute as @e[tag=test,limit=1] at @s run tp @s ~10 ~10 ~10
   

8. /data get entity @e[tag=test2,limit=1] Pos
   

9. Notice the X and Z positions updated but the Y position did not