@caltom Thanks for the report and especially the steps to reproduce. I can reproduce the problem here, and it only occurs with the “Show only free/busy” setting. When the full event details are available, RRULE is used as I would expect it.
I have never seen such events (list of events with same UID and all events have RECURRENCE-ID) and I don’t think this is valid. RFC 5545 22.214.171.124 Recurrence ID says:
Purpose: This property is used in conjunction with the “UID” and “SEQUENCE” properties to identify a specific instance of a recurring “VEVENT”, “VTODO”, or “VJOURNAL” calendar component. The property value is the original value of the “DTSTART” property of the recurrence instance.
So without a main event with DTSTART, RECURRENCE-ID doesn’t make sense. Also, I don’t know how to process those events; after all, they have the same UID, so storing them as different events could cause problems (different events must not have the same UID).
I will further investigate. Did you already have a look in the Google Calendar issue tracker?