I don't think this is related to default calendars or On My Mac vs iCloud, nor is is fixed by upgrading. I upgraded to Sierra last month, and now the issue has been fixed for new events, but the problem still persists for editing existing events, just as it has across three OS's now...
The only reliable solution (across all devices, mobile and desktop, and across the last three OS's I've used) is to make edits in no more than one section of the event entry window at a time. If you edit more than one section of an existing event, only the last edit sticks. it doesn't matter what order you enter text vs changing the calendar... only the last entry is saved.
For instance, if you create a test event (by any method, in any view), type in "APPOINTMENT" for the event title, then enter a location in the Location field (I entered "Test"), then assign it to any calendar you want. If you're running Sierra, everything about your new test event should save just fine even though you've made edits in more than one field before saving (at least mine does). This didn't happen for me until I upgraded to Sierra, but now it does, on destop and mobile.
Now, go back into your test event, change the event title to "Appt", delete whatever you entered into the Location field, and then assign it to a different calendar. When you click out of the entry window, only the new calendar you assigned is saved (the last edit). You'll see the event title revert back to "APPOINTMENT", and you'll see the location revert back to "Test".
If you edit the calendar assignment first, and then the text, only the last text edit will save, and the calendar assignment will revert. This is what leads me to believe it's not related to the default calendar choice or the On My Mac vs iCloud issue so many other users have mentioned.
The ONLY reliable way to make sure none of your calendar data reverts is to make edits in only one field of the entry window at a time. Enter your title, then click out of the entry window to save. Then go back in and add your location, click out and save. Then finally, go back in and assign to your calendar of choice.
It's annoying for sure, but doesn't take much longer than entering everything all at once after you get used to the routine, and it works every time, and on all devices.