MARK AS SOLVED!
Turns out the TRS-TS adapter was working. What made it seem erratic was that I had earlier reverted to factory settings, and a lot of the presets map the expression pedal to certain control knobs by default. This made the usual method of initiating the edit/assignment process using the pedal and the knobs seemed like it wasn’t working, because whatever knob I was choosing was just being added to the mapping for that preset and only the default one showed in the status display. Once I connected to the H9 app via USB I could see what was happening and un-map the default. After that the new M-Audio pedal worked as expected, so yay. I did have to futz with the trim a few times, but that that seemed to magically clear up the second time I did a calibration on it, so I’m getting a nice precise 0-100% heel-toe.
I’m still baffled by the question why my original M-Audio pedal worked fine for so many years without any question of using an adapter. I can only conclude that M-Audio changed the jack from TS to TRS at some point. The TRS connection does seem to dominate among midi-controlled devices—keyboards and the like–and that’s their primary market, so it would make sense. I just wish I’d stumbled on that info sooner if that’s what happened. I’ve posed the question over there but no response yet. May remain a mystery.
But for anyone else who runs into this problem: just get the adapter! Woulda saved me a lot of time and frustration if I’d known.