not gonna pretend to guess at knowing their medical situation, but yeah, some medications (hrt) can do it, or back problems, probably some other stuff too. i've lost a little bit of height myself

estrogen shortens ligaments in a way that can reduce the space between bones and also causes postural changes which can reduce overall height. mine hasn't changed (I've been 5'6" the whole time), but i know people that lost 3"

i don't know jae's age (i suspect they're a little young for this) but it's also totally normal to get shorter as you age - the padding between your bones wears down, your posture changes, your feet get flatter. 'little old man/lady' is an idea in part because people do just get littler when they're old

yeah, de/hydration changes it too

And it's literally because of the disks in your spine compressing under your weight

Astronauts on the space station get measurably taller over the first few days as their spines are unloaded!

5'8 and taller, look it up