In 4.6.0 the handling of '\' in the -U is done at the command line parsing. This means that get_cmdline_auth_info_username() never returns a string with '\' and the domain part is always overwritten by set_cmdline_auth_info_domain(lp_workgroup()).
Created attachment 13132 [details] Patch for v4-6-test
Karolin, please add to Samba 4.6. Thanks!
(In reply to Andreas Schneider from comment #2) Pushed to autobuild-v4-6-test.
(In reply to Karolin Seeger from comment #3) Pushed to v4-6-test. Closing out bug report. Thanks!