User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0
Steps to reproduce:
Just start Thunderbird
You'll get after a little while several reminders you can't dismiss because they correspond to read only remote caldav calendar. You go and open the settings for that calendar, and you notice the "read only" setting is lost. You go and set it back to "read only", and now you can dismiss those old reminders. But if you close TB, and start it over again, once again the calendar comes with the "read only" setting lost, and you have to do the process all over again, every time starting TB.
The remote caldav calendars should keep the settings provided by the user. Whether not read only, or read only, that setting should be kept as per the user configuration, and should be kept across TB turning off and turning back on...