Remember that this is pretty "usual" behaviour for most planes assuming that lateral balance, or other factors are at play.

I don't know how many times I've seen this asked about Ultimate Biplanes...

You may have the elevator adjusted at the moment, to the point that you are getting one wing to stall before the other during the tight loop.

There are two ways to cure this

1. Lessen the loop or throw so that the wings do not stall

2. INCREASE the throws so that both wings stall at the same time.

I usually will adjust the throws so that at low rates I get a nice round loop, and at high rates I get a tight loop with both wings stalling eveningly.

Both adjustments result in round loops.

My Funtana did the same thing as yours until I adjusted it, because of it's large high deflection elevator.