… which was resolved for me, luckily.
The update to A.0118 ran without a problem now, too.
Just to be clear for people reading along … you can’t “flash” TWRP on the Fairphone 3.
You just fastboot boot it … or, if you really want to, you can install it once TWRP is booted, which embeds it into the boot partition using an install script, which breaks OTA updates because the boot partition gets changed this way and the updater checks for such partition changes and doesn’t update in this case.
If installing the TWRP stock packages doesn’t work for you, there are stock partition image dumps out there you can flash with fastboot …