There can be two possible scenarios in this case.
My customer self-terminated on May 9th intending to terminate for May 31st, but they were terminated for June 30th instead.
Notice to terminate on a M2M agreement is due by the first of the month. In order to terminate by May 31, your customer would have needed to process their termination by May 1st. Since they did it on May 9th, the system correctly terminated their agreement effective June 30th. No correction is available.
Please refer to section 1.4 of the Terms and Conditions of the agreement.
My customer self-terminated on May 9th 2020 intending to terminate June 30th 2020, but were terminated for December 2025 instead.
This happens when the M2M agreement could not be entered with the correct M2M feature and is entered as a regular booking with an end date in 5-years. We will still honor the M2M terms of the agreement. If this happens, you can correct the customer termination end date by helping them to contact Account Helpdesk or by raising a booking correction task via Company > Log a task > Booking correction on TeamHub.