This appears to be a normal behavior for Google Calendar reminder and not for an event according to this thread:
I saw that it was ‘Administrator’ that was ‘updating’ the event regularly. So the change is initiated by Google and not by ERPnext.
When using the Google Calendar sync only as a push and having the Pull option un-checked the events won’t move anymore. But than you break te more than usable easy option to move your event by drag and drop from the Google calendar interface.
So the issue is actually in how ERPnext is sending the event to Google as a reminder and not as an event.
I am to new to this to tackle that for now but maybe somebody can point out where to look.