Mistreatment in what way? They were simply weaker than the schutzstaffel and no longer relevant to the plot.
If you think it's okay to not only not let 3/5 members of this incredibly hyped-up squad do much against the Schutzstaffel, but then also have them be off-screened, then I don't know what to say.
Only Ichibei - because he fought Yhwach - and Oetsu - because we saw him cutting down the SS - were treated right. Poor Kirio, Senjumaru and Tenjiro, they deserved better than to be off-screened and not be relevant for the rest of the arc, despite being the Royal Guard of the Soul King.