I just tried it again with the same behavior. A few more details from some testing: When I first open MC, the keyboard buttons operate the Mac System volume, not MC. If I Ctrl-tab to another program, and then back to MC, the keyboard volume buttons operate MC as they should.
Unfortunately, now even though I have restored my settings to a backup from 1/19, I still have this volume keyboard problem when starting MC. I'm not sure why the restore didn't permanently fix as it did yesterday.