Thanks for the report!
However, I'm having a hard time reproducing this myself. From your log I see that the repo seems to have somehow gotten checked out in a detached HEAD state: Checking out Revision aec06608a9427dbb7c643db7368e49260dff5c42 () indicates that there is no branch name, i.e. the parentheses are empty.
Your refspec seems odd as well (no mention of origin), but that appears to be unrelated.
The Git plugin does seem to allow for this situation — i.e. no branch can be associated with the checked-out revision — so even although I can't reproduce this, I can fix this plugin by adding the same sort of behaviour.
It would, however, be nice if I could reproduce your situation, and add an automated test to guard against it. Also, it would just be nice to know what causes this situation to arise in the first place!
Could you possibly grab the config XML for this Jenkins job, i.e. something like https://jenkins.example.com/job/jcpv2-resources-bgm/config.xml and post at least the contents of the <scm> tag here (with the sensitive parts of the URI masked, as above)?