The most common explanation for such short flight times would be batteries that had partially discharged.
Easy to confirm if this was the case by checking the flight data.
The 15 mins you mentioned first sounds abnormal for the Mavic 4 pro.
Your flight data will probably find the reason for it.
Can you give a little explanation for your new "carbon based batteries" ?
Here's some follow-up that I've been working on to show how DJI's "GPS Altitude" is derived from atmospheric pressure and can vary with the weather by +/-200 metres.
For some months, I ran some test flights with a Mavic 3 pro from a known...