Hi Mike,
From looking at the code, the logic seems correct.  While a meeting is active, the only way to allow additional extensions (i.e. reset the current number of extensions to 0) is to (a) modify the extension attributes (which you are clearly not doing by your usage of null parameters), or (b) modify the meeting to explicitly lengthen it (i.e. setting the duration to a larger number).  I cannot see any way how the adding of an endpoint could result in this behavior.  Is it possible that you are doing (b) inadvertently?  Using soapUI on an active meeting, you can easily verify this by setting the duration to null during the modify operation.
Let us know what you discover on this.  Thanks.
- John

Some additional testing has provided the following:
Scenario: Meet-me meeting, scheduled duration = 15 minutes, Meeting Extension Enabled Type = ENABLED, Meeting Extension Period = 15 minutes, Number of Extensions = 1
Active Meeting Management API Modify Meeting parameters: Duration = null, Meeting Extension Enabled Type = null, Meeting Extension Period = null, Number of Extensions = null
 
Case 1: No endpoints added during the meeting >> 1 15-minute extension (as expected)
Case 2: Endpoint added at the 10-minute mark >> 1 15-minute extension (as expected)
Case 3: Endpoint added at the 14-minute mark >> 2 15-minute extensions (1 extra extension)
Case 4: Endpoint added at the 20-minute mark (5 minutes into 1st extension) >> 2 15-minute extensions (1 extra extension)
Case 5:  Endpoint added at the 10-minute mark and at the 20-minute mark (5 minutes into 1st extension) >> 2 15-minute extensions (1 extra extension)
Case 6: Endpoint added at the 10-minute mark and at the 12-minute mark >> 1 15-minute extensions (as expected)
Case 7: Endpoint added at the 20-minute mark (5 minutes into 1st extension) and the 22-minute mark (7 minutes into 1st extension) >> 2 15-minute extensions (1 extra extension)
 
Another group reported:
Meeting extension is added if another endpoint is dialed out to after the notice of meeting extension is seen. For example with 2 minutes left in the scheduled meeting the text saying "your meeting has been extended 15 minutes" is displayed. If an endpoint is dialed out after that, the meeting is extended 15 minutes as expected but then again for a second 15 minutes. If you dial out another endpoint after you are in the first meeting extension you get a third extension. Dropping the endpoint that was dialed out does not cause the meeting extension to be taken away.
 
Can you advise us if when we add an endpoint within the "meeting extension period" (see definition that follows) and specify DISABLE for the Meeting Extension Enabled Type will that stop any further meeting extensions? We observed that once the meeting is at the 2 minute warning the CTX shows the extension being added so for the purposes of this question consider the "meeting extension period" to be any time within 2-minutes of the meeting ending or in meeting extension time.

Some additional scenarios:
****
Scheduled Meeting Duration: 15 mins
Scheduled Meeting extension enabled type: ENABLE
Scheduled Extension period: 15 mins
Scheduled Number of extensions: 2
Active meeting new Duration: null
Active meeting extension enabled type: null if within first 13 mins else DISABLE
Active meeting Extension period: null
Active meeting Number of extensions: null
>> Tried adding endpoint in the first meeting extension at 20mins mark, request fails with message “Cannot change the meeting extension enabled setting for active meeting.”
>>   Tried adding endpoint in the second meeting extension at 40 mins mark, request fails with message “Cannot change the meeting extension enabled setting for active meeting.”
****
Scheduled Meeting Duration: 15 mins
Scheduled Meeting extension enabled type: ENABLE
Scheduled Extension period: 15 mins
Scheduled Number of extensions: 1
Active meeting new Duration: null
Active meeting extension enabled type: null if within first 13 mins else DISABLE
Active meeting Extension period: null
Active meeting Number of extensions: null
>>       Tried adding endpoint in the meeting extension at 20mins mark, request fails with message “Cannot change the meeting extension enabled setting for active meeting.”


****