When an armor stand is summoned and made to change it's x-rotation in the same tick it does not update. This however is not the case for the y-rotation which does update.

Here are a few methods I tried to make the armor stand have the player's rotation:

*Note: All of the following commands are executed in the same tick as when the armor stand is summoned. These commands do work after the tick in which it is summoned.*

1) Using "/execute rotated" 

    /execute as @e[type=armor_stand] at @s rotated as @p run teleport @s ~ ~ ~ ~ ~

2) Directly storing players rotation into the armor stand

    /execute store result entity @e[type=armor_stand,limit=1] Rotation[0] float 1 run data get          entity @p Rotation[0] 1 (works)

    /execute store result entity @e[type=armor_stand,limit=1] Rotation[1] float 1 run data get          entity @p Rotation[0] 1 (does not work)

3) Indirectly storing players rotation into the armor stand using scoreboard  

    /execute store result score y Rotation run data get entity @p Rotation[0] 10

    /execute store result score x Rotation run data get entity @p Rotation[0] 10

    /execute store result entity @e[type=armor_stand,limit=1] Rotation[0] float 0.1 run                  scoreboard players get y Rotation (works)

    /execute store result entity @e[type=armor_stand,limit=1] Rotation[1] float 0.1 run                  scoreboard players get x Rotation (does not work)