Current hypothesis: there are multiple versions / vintages of the Android Auto car-side implementations of the protocol. I suspect that Google has quietly dropped support for one of the older versions and/or they've dropped my vintage of Volkswagen head-unit from their test matrix. This might explain why it mostly works, until it doesn't.

The car, itself, is still in perfect working order and I have no reason to want to upgrade it, except (fun fact!) Volkswagen has dropped manual transmissions from the GTI as of the 2025 model year.

Of course, there are still a few other cars with manual transmissions, and who knows how I'll feel about this by the time my VW finally gives up and dies. But... grumble.