Well, that can't be good. It sounds as if the brakes weren't being released all the way. To the dealer with you!
If it was just a matter of software, then all the ST Fiestas would have the problem. I don't think yours has special software.I think it's a software problem with the hill assist thinking it needs to hold the brakes while I'm slipping the clutch. I am not sure the dealer can do much if there's not a software update available.