But the short summary is that the Honeywell systems used by Airbus and Embraer use baro data to calculate the FPV - if air data is unreliable then FPV is unavailable.
If this is the case, and I believe you, again it shows how complex airbus systems are designed.
I'm pretty sure, on the 1st generation airbus (A300/310) FPV was solely IRS related...
Cheaper, smaller, etc. that's what counts.... Taking in to acount a very complex system which may, and this is the case right here, fail to give appropriate infos to the pilots.