That's embarrassing, my bad.
After the first flash with the wrong twrp version, I ended up with twrp 3.7.0_12-Mi11Ultra_v6.8_A13.
But after flashing miui 14 with this twrp version, getprop ro.build.version.release returns 12.
At the end, if I try to reboot to system, twrp warns me : "No OS...