I think its a glitch or Chester is just feeling so bold that he went to the practice in his underwear. XD
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I think i know what happened tho! when using renpy (the engine this is on), if you wanna display a character you use the following line
show ______
usually you would classify outfit and expression with this as well.
however due to the art change dyne has added new poses expressions and outfits.
so I believe before the change his show line was
"show chester"
instead of
"show chester (Outfit and expression)"
hopefully this will be fixed lol