There is no aerodynamic reason for your model to do what it's doing outside of a physical misalignment of some form that is creating a trim condition that causes this. I'd suggest you start with a full on alignment/setup survey of your new bitsa model. Ensure that the fin is aligned to a center line and from there check all the other aspects that would cause a roll like wing skew and side thrust angle. One item that may be present that you didn't allow for is that the wing saddle is angled differently side to side. Then when you clamp the wing on it would induce a twist in the wing that may not be there when it's off the model.

There's lots of tricks to alignment checking your model. the old thread or string to check for wing skew, etc. It's a long process where each step relies on the results of the one before it.