Fitbit Air inaccuracy

I use Fitbit Air as a daily driver with Apple Watch Ultra 3 as my fitness watch.

I've noticed that Air are having some HR inaccuracy here and there vs AWU3 (even with HR chest strap)

- too low HR during workouts - e.g. AWU3/H10 - 120 vs Fitbit Air 56

- too high HR during workout - e.g. AWU3/H10 - 160 vs Fitbit Air 183

this happens quite often even when sitting/ressting mode and when I accidentally wear both devices I see that there is HR inconsistency by sometimes 10bpm at the same time

I recon this happens after the last firmware updates, because I do also lost naps