Breakneck Blitz does not apply damage properly with the move Return

[Ariesden 0]

I was battling a trainer in the  grand hall with my Lopunny against a Sawk testing the Normallium Z with the move return, the problem is when i used Breakneck Blitz (with return), the move dealt the same amount of damage as if i was not using the Z-move at all and just used normal Return. 

 

I then reseted and used the move Return normally to double check this, and both dealt the same amount of damage. Maybe it is because the move varies in power, if so, it could happen with other power-varying moves as well, like Heavy slam or Grass Knot. 

 

I tested Breakneck Blitz using the move Strength and it worked properly, i dont know if that helps