Staad.Pro creates a moment load on Self-weight

Hi, I'm having issues with a plate model on Staad.

I've put as "Selfweigth" a multiplier of -1 for global Y on all plates, but when I analize it, on the results section appears to exist a moment load, and the output for base pressure is weird.

Output for base pressure on selfweigth case (i suppose it's a bug or something, because the values are all 0.072):

Moments on selfweigth:

I attach the model here in case anyone can help me with this.Platea.std

Thanks in advance!