Considering he's gotten in an endless loop of deployment > injury > hospitalization > therapy. Him losing muscle mass is surprisingly quite realistic, but the reason behind his muscle loss is most likely inconsistency in art, and not the artist deliberately wanna depict realistic muscle loss (methinks)
Though imma yap a bit more, he still have his muscles but those are functional muscles built by constant fighting for 10 years, meaning those muscles are built from constant usage, while before, he had aesthetic muscles which are built by workout routines targetting certain muscle groups.
Fun fact: functional muscles are far superior than aesthetic muscles, best example is how rock climbers doesn't often have the biggest physique as bodybuilders, but they can most certainly lift weights as heavy (if not heavier) than what bodybuilders lift.
*Learned this from my gym coach who's a rock climber ヾ(☆▽☆)
Why does the bottom kinda look different am I tripping or did his buffness just disappear