I’ve switched this over to the bug category as I’m now fairly sure this is in fact a bug in Discourse.
These two posts replying to the instructions for setting up Discourse Connect describe the same problem I’m having:
The first of those two posts has the problem staring after updating to 3.5.0.beta5-dev in late May. I had already identified the problem as starting for us no earlier than May, which I suspect is not coincidental.