Maker API stopped POSTing device events

I've had my Hubitat C-7 integrating with another automation system for a while, functioning by using the Maker API to POST device events (e.g. motion detections, or controller button presses). That's been working for months (or maybe years now?), but at some point over the last week or two it abruptly stopped working. I don't know the exact timing, but it might have coincided with upgrading from 2.3.8.125 to 2.3.8.133, or from 2.3.8.133 to 2.3.8.134.

If I query the Maker API directly (e.g. GET to /apps/api//devices//events?...) then I can see the events immediately after they happen, so I know the events are happening and are making it to the Maker API. What is not happening is the Maker API sending POSTs to my configured endpoint. I've tried the Enable Logging for debugging, and see the device event: ... line, but nothing else happens, in the logs or elsewhere.

It might be the same issue as @alfanerd had in December (I'm not allowed to link to that thread?!) and I've ruled out things like the destination address being wrong (I'd expect the Maker API to log an error if it failed to POST due to that, right?). I also tried adding a new instance of the Maker API, and that didn't help either.

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