I just tested and the months do translate with the new POT file. I'm guessing it's failing for some because the new POT references new calls in the other plugin files. In other words, Kaj is right in that it's best to just get the new POT in a release along with everything else.
Thanks for bearing with us as we try to figure out translation for the first time.
I noticed in the .pot and subsequent .po files, each entry of a term has a line number associated with it. When we make updates to the plugin files, these line numbers will inevitably change. I'm about to test whether or not this would introduce translation bugs. Does anyone already know?
EDIT: I tested cases where the .po line numbers do not match with the gettext calls in the plugin files, and confirmed that the translations in the .po file still work properly. This means that translators only have to update .po and .mo files when we release a new .pot file, not every time we fix a bug. Sorry for the didactic spiel if you already knew this.