Not a bug in pbx, but a feature in the new b2sserver.
This is to put pinupplayer to front (so b2s is not on top anymore, but will hide behind pinup player)
Because pbx has a black screen (to hide icons) it will show the b2sserver behind that screen.
Maybe an option is to check if pinupplayer is runnong, if not, put the b2sserver exe to front? Not sure if that is possible for you ?
I'm using the new b2sserver for a while now and uses Scutters solution.