Shifting Rune and Fighter Proficiency

[PochiPooom]

¿If a fighter that choose "Hammer" for his highest proficiency is using a shifting rune does he lose his "Fighting Weapon Mastery"?

Shifting Rune: https://2e.aonprd.com/Equipment.aspx?ID=302

[Ascalaphus]

Yes.

You're very good at hammers, but if your weapon isn't shaped like a hammer that's not good for you.

[Aw3som3-117]

I agree with Ascalaphus. It becomes a different weapon. The fact that magic is causing it to change doesn't really change that.