I have no idea, but: are we sure that there are no GUI plugins involved?
will check again - but i'm pretty sure since he's not a pd user. there might be some remains from a previous external test i did (don't remember), but that wouldn't have involved any GUI plugins.
go through some old builds and try to figure out which commit broke it.
i'll first quickly try to reproduce it on my own system since he unfortunately needs his computer for work now. ;) ... but i might be able to do this these days!
cheers, ben