Notice if an automatic update to FP3-REL-Q-3.A.0054 fails with such a routed phone. Download FP3-REL-Q-3.A.0054-20200929.092252-user-fastbootimage.7z then unzip with 7z - put Fairphone3 in fastboot and FP3_flashall
then it is in the new version. Magisk will then no longer work. So after successful flashing go into fastboot again. Then fastboot boot twrp-3.4.0-0-FP3.img
Enter its pin on the screen to decrypt the partition.
adb push Magisk-v21.0.zip /sdcard/Download/
Install Magisk with twrp and maybe even props
After the restart everything seems to be ok, but apps at least my targobankapp recognize the phone as rooted, so it is not so, execute su props and force as above and of course hide again. After that everything was ok again.