First and foremost the cheapest path is to drain and refill with LifeGuard 6 or Mercon SP. I have found that BMW techs get interrupted at times and fail to correctly fill the transmission. Also this transmission level is checked engine running in park and opening the fill plug. Fluid should weap out. If not it is low.
I would have an indy do that since it is the cheapest path.
If you don't have a transmission leak and the level is correct then a failing transmission is possible but a drain and refill is inexpensive.
Good luck and please post what is discovered...