From version 1.15-pre1 rotation and position of armor stands have been completely broken. It is similar to MC-79735, but much more intense. Bug ocures when moving/rotating an armor stand using tp command or execute store. Here you can see how to reproduce it, and here is a comparison to 1.14. This thread is similar to MC-166132, but that bug mentions only tp facing.

Examples of commands required to reproduce the bug:

1. Simple teleportation:
   /execute as @e[type=armor_stand] at @s run tp @s ~ ~ ~ ~10 ~
2. Execute store:
   /scoreboard objectives add time dummy
   /scoreboard players add @e[type=armor_stand] time 10
   /execute as @e[type=armor_stand] store result entity @s Rotation[0] float 1 run scoreboard players get @s time