Another update!
At my follow-up with the surgeon after the February EUA, she said they couldn't find a fistula. I asked her flat out "Do they think its there but they can't find it, or do they think there isn't one?" She said they don't believe I have one. Pretty peeved to hear that, and felt very dismissed.
She gave me some cream to try *eye roll* and said to come back in six weeks. So at the next visit I brought photos of toilet tissue showing the discharge (gross but effective). Finally it clicked with them that I wasn't making this up. I also asked whether an MRI was worth doing. Surgeon thought yes, and it highlighted an area for them to focus.
Scheduled another EUA, done yesterday, and wait for it, they found the fistula! Five years of stealthily evading detection, and he's been found. Apparently quite low down in both the pouch end and the vaginal end. Surgeon is recommending a fix through the vaginal side. (Only had a 2 min conversation in recovery, will found out more at follow-up appointment).
Also, I got a surprise seton! I had no idea this was on the cards, I thought the EUA was just a look-see. Luckily I knew what a seton was and its purpose, so I didn't freak out too much. But, a little heads up would've been nice, doc!