Google Calendar also produces ICS files that exhibits this behaviour.
Steps to reproduceCreate a calendar in Google Calendar. Add an event that repeats every week. Set the calendar to “Make available to public” and “Show only free/busy (hide details)”. Load calendar in ICSx5 (which I did via HTTP)
ICSx5 displays an event every week.
ICSx5 display no events.
I have attached the ICS file. basic.ics
It’d be great if ICSx5 were liberal in the input it accepts! I can look into this if desired.
The relevant code in ical4android appears to be https://gitlab.com/bitfireAT/ical4android/-/blob/7f263010549f13098d02da8512f89cb02a4cb829/src/main/java/at/bitfire/ical4android/Event.kt#L131-141 . All the events in exceptions that have no parent in mainEvents will be unprocessed, and thus not added events.
RFC 5545 appears to be ambiguous as to how to handle this situation. I was unable to find anything that stated that events with a RECURRENCE-ID must have an RRULE or RDATE.