Have never come across the word before either. Makes you think - did the person say the word to convey some kind of meaning? Or to sound super-smart to their peers?
Even "disharmony" is far more common than "disharmonious."
"And a sea of pulchritudinous models would have served as a lesson on how not to choose words: the ugly pulchritude sounds like the opposite of what it means, and it is one of those words that no one ever uses unless they are trying to show off.” https://www.goodreads.com/work/quotes/40167209-the-sense-of-...
That said, as a french speaker I would have had no idea what this word means if I did not took Latin in high-school.