The bug

With the new /execute command syntax, the subcommand at should change the command's position, rotation, and dimension to the targets specified. However only the position and dimension are set; one must use /execute as to set the rotation, which does not line up with behavior in Java Edition.

In Java Edition, /execute as solely changes the targets executing the command and it creates a new command stack for each target. Position, rotation, and dimension are not inherited from a target this way.

Steps to reproduce:

• Create a new world with the Upcoming Creator Features experimental enabled.
• Place down an armor stand.
• Run the following two commands and keep an eye out for where the particle is emitted at.

• /execute at @e[type=armor_stand,c=1] run particle minecraft:large_explosion ^ ^ ^3
  

→ The particle emits relative to the armor stand's position, but inherits your rotation.

• /execute as @e[type=armor_stand,c=1] run particle minecraft:large_explosion ^ ^ ^3
  

→ The particle emits relative to your position, but inherits the armor stand's rotation.