I've used the workaround in other places, but it's not working for me here:
https://steamcommunity.com/sharedfiles/filedetails/?id=2824641135
This inserter should be active, as its control behavior was copied from an adjacent UF Filter Inserter, which I tested as configured correctly, but ... no luck.
[UPDATE]
I think I figured it out. If I remove all circuitry, I see the message "waiting for train".
The adjacency to the train track makes it think it's inserting into the track, when (due to configurable distance) it's not.
[UPDATE2]
While that's definitely an issue, it's not the only one. No matter what, I can't seem to make my pictured UF CF Inserter work, if it's connected to the circuit network. Adjusting the position, even changing the angle and being nowhere near a track, nothing fixes it. If it's got a circuit rule on it, it doesn't work at all.