Some AC50 teams explained that they set their boat for upwind which brings the bow down attitude downwind where they need less AoA at faster speed.
TNZ had also a bow down attitude upwind, one explanation could be that they felt confortable flying upwind in this configuration allowing a better AoA, thus better stability when the boat went slower during tacks for exemple. How could they keep the same setting downwind ? don't know, sail trimming perhaps.
I have not seen this bow down attitude with F50, probably because they have a different rule allowing better control with the rudder rake.
I have not seen this bow down attitude with Ineos and AM either which would be logical as they have main foil flaps, they are sometimes bow down but it doesn't seem that they are loocking for it but lacking some power when it happens.