Seems the only way to actually alter the crank length is through a secret menu on the app?
You can maybe try that to double check it is set correctly but if it is then I’d just put the fenix reporting the wrong value as an annoying bug.
It shouldn’t effect the reported power as I don’t think the watch has the ability to change the value on the power meter.
Hopefully someone else will chime in with non 172.5 stages and a fenix and let you know what they see.