Since it was a bug it's not 100% guaranteed to work every time, sometimes it took us a few tries to get the pop but generally not many. From the looks of it it's mainly the US having problems so maybe it's been previously fixed or something? I'm not sure exactly how old this glitch is.
Yeah that seemed normal, nobody can be Q'd for BGs or anything either I forgot to mention sorry. Also once everyone is in and roles are confirmed the dungeon eye icon thingymajig turns to a yellow "..." and when you hover it it says "< 1 minute" or w/e then pops seconds after. If not then disband group and retry. Not played random dungeons for a few patches so not really sure if that's new/normal
Edit: Oh wait thought you said "even though it let us select roles"...Not really sure then, for us it let's the first 2 guys who join random to pick their roles then when you invite the others they too can pick roles when they accept the invite.
pretty sure this doesnt work for US, i have 30/34 (both dps) 85(heals) me(tank) twink friend (dps) 30s join group and Q inv 85, then inv us and dungeon finder drops us
Twinking is about making sure no one has an advantage you do not, if one person can make it work, well then everyone is going to do it best to have a head start.