The description of the feign death spell says, in part (PHB, p. 240):
If the target is diseased or poisoned when you cast the spell, or becomes diseased or poisoned while under the spell’s effect, the disease and poison have no effect until the spell ends.
Part of the description of the harm spell says:
If the target fails the saving throw, its hit point maximum is reduced for 1 hour by an amount equal to the necrotic damage it took. Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.
Here is a hypothetical scenario:
- Have harm cast on a player character (reducing the target's maximum hit points)
- Cast feign death on the character, suppressing (not removing) the effects of the disease (and thereby removing the reduction to the character's maximum hit points)
- Restore missing hit points (e.g. via cure wounds, now that their maximum hit points are no longer reduced)
- Wake them up (i.e. cancel feign death)
When feign death ends, does the character's hit points stay as they are (at their regular hit point maximum)? Or are their hit points reduced back down (effectively taking damage again)?