Nemo_bis's words: "It's either from CLDR or PHP which gets it from ICU which gets it from CLDR."

It's possible it's in CLDR or the Wikimedia code somewhere; I just don't know where to pinpoint it, exactly. It's not in Mediawiki:Month-gen/oc, as a) all twelve have that 'de' and b) the Occitan wiki currently has a bug where the "de" before the month is *not* included (using 'long' by mistake). So something is written DE junh DE.

The extra "de" is mostly confined to Wikipedia; it appears once on the Wiktionary landing page, but the Commons and the rest of Wiktionary are unaffected by the June bug.