For your reference - Last week, after last Magisk update, my FP4 started to malfunction: Magisk application disappeared after the installation and no more wifi, nor sound. I wanted to go back and manually flash stock FP4 ROM but, after a series of manipulations and re-locked the bootloader, I received the message "Your device is corrupt. It can’t be trusted and will not boot” and I only have access to the Fastboot mode.
I have just contacted support to have them reset the phone to factory defaults…