If you store the score of an entity into its Pos data it won't update properly. If you run /setblock at the entity it will put the block at the position you stored from the score but the armorstand will visually stay at its non-updated position for 30 - 60 seconds. Then instantly snaps to its new position.

Here  a video of the bug: https://www.youtube.com/watch?v=D-0ICNyf2II

How to replicate:

First create a scoreboard called "bug.y". Then you need to setup a Datapack or a Chain Commandblock contraption. Now put the following Commands in that specific order into the Datapack/Chain Commandblock Contraption.

1. summon armor_stand ~ ~1 ~

2. scoreboard players set @e[type=minecraft:armor_stand,tag=bug_tester] bug.y 60

3. execute as @e[type=minecraft:armor_stand,tag=bug_tester] store result entity @s Pos[1] double 1 run scoreboard players get @s bug.y