Seals in the PTO is the most likely culprit. Get the quad rings instead of orings when reassembling. Supposedly the quad rings are supposed to come up when looking up parts for it, but the last one I did had one quad and one oring show up.

Doesn't happen often and I only know of one person it happened to, but the metal stamping that separates the hydraulic housing from the transmission housing and acts as a reservoir, had a crack in it and was leaking hyd oil in to the gear oil.