Jump started dead car, after 5 minutes I removed the negative jump lead and the car died again, why? Positive was still connected

The battery health indicator is green on the dead car, when running it was 13.8v

Kept the previously dead car running, turned the ignition off the working car but when I removed the negative cable only the car died.

What is happening? C

Mini Ccoper S 2006 1.6 Tritex