Astral Guard not only decreases growth of opponents powers, but if there is two of them in a game it decreases current powers as well.

That is if I have Earth=3, opponent has 1 Astral Guard and casts another one, then when turn comes to me Earth power is decreased by 1. Earth=2

that's not a bug; it's how they're intended to work. 

 If so then maybe it should be written: "decreases opponents powers" but not "growth" of powers. Or you think growth can be negative?? eg. "-1"