Had the same thing on my 2016 GSA - no rear brake at all. Dealer fixed it with a flush/bleed, it happened to coincide with the three-year service. It was WAY out of warranty, so I'm sure I rode out with the same master cylinder I arrived with. It's one of those things they won't just do as a goodwill item and not tell you about. Parts fiche from Bob's BMW shows it as $217 and change (plus labor, of course).
Unrelated, I just noticed that both places I check parts prices (Max BMW and Bob's BMW) haven't updated their parts fiches for quite a while - for the R1200 GSA there's nothing newer than MY 2014 - so no R1250 parts. Has the mothership decided to restrict current information to only professional parts-looker-uppers, i.e. the ones employed in authorized service facilities?
Current: 2020 R1200 GSA Low
Previous: 2016 R1200 GSA Low (traded); 2015 R1200 RT (traded); 2012 K1600 GTL (traded); 2011 R1200 RT (totaled, RIP)