Not 100% sure this is a bug, but afaict mappings for the BUILTIN domain are perfectly fine and should not be deleted by `net groupmap cleanup`. Have patch, need bugnumber.
Created attachment 13648 [details] Patch for 4.6 and 4.7 cherry-picked from master
(In reply to Ralph Böhme from comment #1) Pushed to autobuild-v4-{7,6}-test.
(In reply to Karolin Seeger from comment #2) Pushed to both branches. Closing out bug report. Thanks!