Exchange allowing conflicting meetings when it shouldn't

Question

I have a client that recently migrated to Exchange 2013 from Exchange 2007.  Shortly before the migration, I converted all their resource calendars to the Resource Booking Attendant booking method (AutomateProcessing setting is set to AutoAccept).  They
do not want any conflicts to be allowed, so I set the following settings on all the resource mailboxes:
AllowConflicts - False
ConflictPercentageAllowed - 0
MaximumConflictInstances - 0
I have tested and determined that these settings seem to work properly, at least for new meeting requests.  However, there seem to be some scenarios in which recurring meeting requests are approved even when there are several conflicts.
After some investigation, my suspicion is that existing recurring meetings that originally had no conflicts when accepted (and likely were accepted prior to enabling the Resource Booking Attendant) are automatically accepted when the original series is modified
to extend the number of recurring instances, even if the new instances are in conflict with other meetings on the calendar.
Can anyone tell me why Exchange may be allowing updates to existing recurring meetings that cause conflicts?  Based on everything I've read online, the current calendar property settings noted above should be preventing this.  I need to find a
way to prevent this, or at least be able to explain to the client why this is happening.

Answer 1

Yes, I've found that article linked to many times, but this seems to be beyond what's covered in that article.  Here are my responses to each of those methods:
1) Send Follow-Up Nonrecurring Meeting Requests to Double Book
The meetings in question that are being allowed to double-book are recurring meetings, so this does not apply.
2) Allow a Delegate to Double Book Resources
There are no delegates listed on any of these calendars, which is intentional, as the client wants management to be automated.  Instead of delegates, a select few users have been granted Editor rights directly on the Calendar folder via an AD security
group in order to make manual changes.  And just to note, the users whose updated meetings are being allowed despite conflicts are NOT in the security group, and do not have Editor rights to the calendar.
3) Send a Series Update Without Changing Details
This actually sounds exactly like what's happening, except for one detail, namely that it states that in order for this method to work, conflicts for non-recurring meetings must be allowed by setting the AllowConflicts property to true.  In my case,
the mailboxes in question all have that property set to False, yet the conflicts are still allowed for some reason.
Any other thoughts?  If this is by design, then if nothing else, I'd at least like to be pointed to documentation that states that it's by design, so I can convey that limitation to my client.