one way to test might be to check character screen mounted and unmounted if physical mastery goes up when mounted, then the war steed strength buff to phys mastery only works when mounted.
I can't get in game to test unfortunately, I'm assuming if you hover your mouse over the war steed panel it actually says war steed strength adds to phys mastery in first place.
IMO it should only buff mastery when mounted as to only affect bow skills when mounted, and imagine it works like that. Maybe someone smarter than I knows
Elendilmir - 95 Hunter Berenthalion - 80ish Burg Berendybuck - baby warden Berenion.
Worst Reaver on the server BerendyBash - R4