You don't really know that "it is working fine"... to you, based on driving it, there are no noticeable symptoms.

There could be 100 things that cause a 'glitch' and trip the DME to register a tranny data fault. Bring it to a QUALIFIED mechanic with a BMW GT1 computer and have them run a diagnostic. If it is under warranty, dealer- if not, elsewhere- unless you don't mind spending money needlessly.