The leave and re-join works during the week. During registration it can re-trigger the problem. Did these members leave/join after the guild had hit Register? Ideally, a guild should wait to Register until after any roster changes if possible.
There's alot of details (and two possible workarounds) in this thread:
Also, the affected players should take screenshots of their participation in the GW battle logs before these get reset. This will be useful when contacting Customer Service. They should be able to get compensation for last week, but the way CS usually works, the burden of proof is on the player.