Update 28 / 11 /21
After doing the lasted -Syu which changes
Replace linux-raspberrypi with core/linux-rpi-legacy? [Y/n] (haven't got a clue what that does or means......
The flickering screen is back again
You need to re do
I modified by /boot/config/txt
from
dtoverlay=vc4-kms-v3d
to
dtoverlay=vc4-fkms-v3d
As it's obviously reset to default with the kernel rename / modif
All good again after