I identified the problem to be this mixin over here

Notably this causes a conflict with unicopia's mixin over here which is supposed to permit pegasi to walk on powder snow, but does not work with Frostiful is installed.

I suggest removing the cir.setReturnValue(false); call as that is preventing other injections after yours from running. The flip side is that if another mod doing this runs before you, they will cause a similar conflict in frostiful.

Thabk you!