- in fact it's known that the FINAL APP mode won't capture on most line selected RNV approaches unless you are at platform on these aircraft..
That's because you
need to be at platform altitude - if you're not at platform you're not flying the approach correctly. There's no strange phenominen here just incorrect technique.
If you look at the coding for a typical RNAV approach, which should always be checked against the published procedure for accuracy to ensure "the box" does what the the plate tells you should be doing, below the platform line i.e. FD09 there will be the final approach flight path angle i.e. -3.1°.
If you are not at platform, how can the box sequence the command to descend?
Modern FMS / autopilot systems are very reliable - provided they are operated correctly. We've all heard of automation complacency, mode error etc. Coupled NPA/RNAV/GNSS/Managed approaches are still relatively new and it is encumbent upon every pilot who operates a modern type to ensure they fully understand how their particular ship goes about flying these approaches. Sounds obvious but there is still confusion out there.
A4