There were quite a few changes to HAP-python that I hadn’t kept up with in my MQTT to HomeKit bridge, but after restarting my computer, I must have updated the installed version in that package, and all sorts of things stopped working.
I spent some time getting code to actually execute again, but had an issue where it was still not working. All of the code was running as expected, but HomeKit was just failing to recognise anything. So, I unpaired and attempted to re-pair.
It failed to pair.
Well, technically, it paired, but then unpaired immediately.
It turns out that if the JSON data that is sent to HomeKit in invalid (semantically, in this case: it was valid JSON data, just not quite valid HAP data), then it will unpair - if the device is already paired, it will just appear as unavailable.
I had some custom code that built up the Information Services slightly differently, but my method of ignoring the standard HAP-python code that added this seemed to no longer work, so my bridge, and all of my accessories had two Information Services.
Fixing this meant that I was able to pair correctly again.