I've got a dozen strops, like any normal person, :p and I've never had one with "ripples/creases". I would think that would be a problem and I would take a shot at the bottle roll over with some pressure to try and flatten them if nothing else. The lather on the strop/bottle method is an old barber thing. I've never tried it either but here is a thread with pros and cons if you want to investigate it further. Posts by 'honedright' and Neil Miller are particularly instructive though on opposite sides of the method.
When I strop I have enough pressure on the spine to keep the razor on the surface and do the flip in a way that the edge just skims the surface with no pressure. I also do an X of sorts since I use 2 1/2" strops. Also, you say "semi taut". Try taut as you can get and see if that doesn't help. Not sure that it will. Worst case scenario, get a new strop.