The Bug

When certain nbt tags are set to the default values, for example: {Pose:{Head:[0f,0f,0f]}} for and armor stand, it disappears entirely, making it unable to execute store. 

How to reproduce

execute as @e[type=armor_stand] at @s store result entity @s Rotation[0] float 1 run data get entity @p Rotation[0] 1
execute as @e[type=armor_stand] at @s store result entity @s Pose.Head[0] float 1 run data get entity @p Rotation[1] 1

These 2 commands looped using repeating command blocks or ticked functions make an armor stand mimic the nearest player's head movement. When I look up, it should look up too, and it does. But as soon as I turn my head down and pass a pitch of 0, the vertical movement of the armor stand stops completely. The nbt is deleted, and execute store stops working.

This will start working again after merging an alternate data other than {Pose:{Head:[0f,0f,0f]}}.