Resolved! 6 out of 11 boards eventually reset right after starting.
I have 11 boards of our prototype with the STM32L4A6AGI6 (i.e. 169 BGA) assembled. 5 of them work fine. The other 6 eventually loose debugging connection. It doesn't happen immediatelly i.e. I'm still able to run a few lines of code but then it reset...