As has been reported, Full Moon appears to permanently decrease the ATK of your allies when it wears off. I've done a moderate amount of experimenting with this, and I'm pretty sure the problem is not that the ATK is getting set to zero, but rather than 2 ATK is getting subtracted twice from the creature, as evidenced by the fact that a Plasma Behemoth ends up at 1 ATK (5 - 2 - 2) rather than 0 ATK.

related: I can't replicate the "Keldor goes to zero attack" bug on the test server without the use of Full Moon, which means I think Keldor is OK?