The new setup with radicale is such that there is a ‘socket’ unit that always keeps listening for new connections and a ‘service’ unit that may (not verified yet) automatically shutdown when not being used. As soon a new connection arrives on /radicale/ URL, systemd should automatically start the radicale service and pass on the incoming connection. All this work transparently to reduce resource usage when services are not being used. Users should not notice it and for them it appears as if service is always running.
But it looks like things are not working as expected. Since you are able to consistently reproduce the issue after a day, I will try to do the same and find out what’s going wrong.
Yesterday, what made me notice that radicale was not running was a failed diagnostic. Today, it was that I had evolution fail to access radicale. Both times, on the plinth page for radicale, there was a notification that the service was inactive.
This morning, none of my clients complain so radicale probably works but on the plinth page for radicale, there is the warning message that radicale is not working.