I’m using DAVDroid with owncloud 10.0.7 with the Contacts and Calendar plugin. The Calendar plugin creates a read-only calendar for birthdays extracted from the contacts. Now there seems to be a problem with contacts for which only a birthdate without a year is entered. The respective vcard contains an event ending with an asterisk *:
All the birthdays where I did not enter a birth year do not show up e.g. in the Google Calendar app or in the Simple Calendar app (https://simplemobiletools.github.io/).
Although I am not 100% sure whether the problem is on DAVDroid’s side or owncloud’s or the clients’, I noticed that as soon as I enter a year for the birthday in contacts, the respective event is shown in the calendars.
When I synchronize the birthday calendar with Thunderbird’s DAV implementation (or Lightning’s), I can see all the birthday events, regardless of whether they have a year associated with them or not.