I'm assuming the second ESC had the same programming as the first. Check the Low Voltage Cutoff. At the end of the flight, voltage can really sag, especially on verticals. Check the Data Log to see the minimum voltage during the flight and make sure the LVC is lower. Although, I would think you would get an alarm code if the LVC was the culprit. It sounds like the ESC is shutting down. You might be able to trace it back to batteries that are the end of their life cycle, since you had some good flights.

​​​​​​​Steve