As @slyecho noted above, it would be advisable to try flashing the reverse-engineered version of the keyboard firmware. Even if you end up with using 50% of the available writes (four out of eight), it would be worth a try, IMHO.
Edit: I would strongly suggest that you try using "showkey -s" and "showkey -k" on a virtual console to capture the scan codes and keycodes, respectively, for the troublesome Fn keys. Then, we can compare those codes with the ones recorded for the same keys on my PineBook Pro.
Edit: I would strongly suggest that you try using "showkey -s" and "showkey -k" on a virtual console to capture the scan codes and keycodes, respectively, for the troublesome Fn keys. Then, we can compare those codes with the ones recorded for the same keys on my PineBook Pro.