How to reproduce:
Have User1 create a public event in Thunderbird/Lightning
Have User2 sync User1’s shared calendar (read only) using Davdroid
Have User1 change the event from public to private
Have User2 sync again
Expected result: The event is no longer visible to User1 and is marked <private>.
Actual result: The event stays visible to User 1.
In the logs we can see that Davdroid is correctly notified that the event has changed, so there is a GET request downloading the changed event which looks like this:
PRODID:-//Alt-N Technologies Ltd//MDaemon 17.5.2
TZID:W. Europe Standard Time
TZNAME:Daylight Savings Time
DTSTART;TZID=W. Europe Standard Time:20180319T140000
DTEND;TZID=W. Europe Standard Time:20180319T150000
One can clearly see that the (updated) event contains CLASS:PRIVATE.
Unsubscribing and resubscribing the calendar correctly shows it as private… obviously…