I just logged into my level 60 rogue on Benediction and I got the feat of strength Soul of Iron (Season of Mastery) despite this character not being on a SOM server. Earlier I got the Bloodthirsty Berserker despite not being in EOTS and was instead in another bg.
The Soul of Iron achieve appears to be happening to all lvl 60 characters. I got it on a lvl 60 Draenei and a lvl 60 with the Tabard of the Protector from TBC pre-patch, both of which are very obvious impossibilities.