When creating an armor stand:
1./summon armor_stand ~ ~ ~ {Pose:{Head:[30.0f,0.0f,0.0f]}}
And teleport its rotation:
2./teleport @e[type=armor_stand] ~ ~ ~ 60 0

it would summon an armor stand with Pose nbt.
But if type this command:
3./data merge entity @e[type=armor_stand,limit=1,sort=nearest] {Pose:{Head:[0.0f,0.0f,0.0f]}}

Althought the armor stand would change its head rotation display to 0 0 0, we cannot type this command to change its head display to as the same as its rotation(the second command change its rotation) because the Pose nbt is gone:
4./execute as @e[type=armor_stand] store result entity @s Pose.Head[0] float 0.1 run data get entity @s Rotation[1] 10

The reason why it type Head[0] and Rotation[1] is due to the bug https://bugs.mojang.com/browse/MC-122057