Transmigrator who has been on anti depressants for years suddenly gets dropped into a situation where they physically cannot get their medicine. But because they been on their meds for years they are like "nah it's fine I'm so healthy anyways".
And then to no one shock other than their own the symptoms come back but unfortunately it happens throughout the plot and you watch them slowly degrade without realizing it until they are back to the state that made them start meds in the first place.
Brownie points if their symptoms fuck up the plot and also negatively effect the people around them.