The servers are still well the same old #serverlimitations bs. You can't talk to anybody except party, guild chat & LS while inside any duty, you cannot access guild roster, fc list, friend list, etc.
I'm not fully sure how the game handles this but if we can't PM while in duty in game the app won't be able to either since they are blocked off for some weird reason (why did they even make it like this in the first place?).
Not even sure why the app won't allow us to talk in guild chat, LS chat, etc either. It's not the app though it's the ancient engine / code of the game. This mobile app will have better functioning code then what the actual servers have where the game is build upon.