I've come across some seeds where Fee is a Pegasus Knight despite Erinys being a Dracoknight. However, I think the issue has something to do with Erinys, since in the almost 60 seeds I've generated from initial FE4 support release, I have not once seen her randomize into a Pegasus Knight. It took a while for Fee to be able to randomize into a Pegasus Knight as well - probably around 0.7.6f or 8.0.

I know you don't allow units to randomize into the same class as their vanilla class, unless they have some other kind of restriction (ie. must be mounted or must be a flier), so I'm confused as to what could be causing Erinys to never become a Pegasus Knight. Particularly when Fee is one. Perhaps somewhere it believes that Erinys is also one, but she's changed later?