Mbed OS 6.12.0 KO, Mbed OS 6.11.0 OK

Hi,
I have just upgraded the Mbed OS library from 6.11.0 to 6.12.0 on my STM32H743VIT6 BoringTech board and it does not boot anymore.
When I come back to 6.11.0 it works again (done several times).
Using a modified system_clock.c file what is the difference between the two libraries at boot?
Any idea?

When I am using the old PinNames.h and PeripheralPins.c files it works. Something not compatible with my board. I will investigate later.