dunno. Style and pose, I guess. ^^ As I said, I understand it's supposed to be ambiguous, but something about it makes me more inclined to go "there's a guy under there".
I'm more inclined to say "There's a butch under there" if you'd ask me! :P In any case, it's suppose to be ambiguous. If they showed anything on the more 'feminine' side of things with the doctor's art, like a 'feminine pose' or indication of breasts, then it kind of stops being ambiguous, no?
I'm glad to see more Arknights stuff here. The series is ripe for yuri content. Angst with Scavenger trying to get over her past girlfriend, Rhine's labs has Saria/Silence and Mayer/Magellen and all of them get to be Ifrit's moms, Texas/Lappland angst with Texas/Exusiai good times, Projekt Red, her weird trait to terrify lupos, her love of fluffy tails, and any other Lupo gal operator, and the list goes on!