I`m not sure if this is a bug, but anyways...

when using "/execute as <entity>" command,

any command containing the coordinate "~ ~ ~" is set as the coordinate of the location of the entity that was previously specified.

But, only the /teleport command sets the coordinate "~ ~ ~" as the player`s coordinate who is using this command.

example: /execute as @e[tag=armor_stand] run tp ~ ~1 ~

=> teleports this armor stand 1 block up of the player using this.

tested in only 1.15.2, yet.