As I understand it this shouldn’t happen anymore per the announcement & changes made to costumes in V28 (March 2020).
However while doing the class trials I changed Sartana from “costume” version to “regular” version (Fighter → Wizard) in order to participate in the Trials of Mysticism.
There was no “warning” dialogue when I went to change the costume version as Sartana isn’t currently part of any of my teams, except on my war defence team.
In the PREVIOUS war, Sartana was in costume form. I am 100% certain of it.
THIS war however, Sartana is in WIZARD form… and I am also certain that I never once went and changed the setup…
What I THINK happened was that changing the costume for the class trials while the war defence team wasn’t locked resulted in Sartana being changed in my War Defence Team.