Forge 31.2.47
Minestuck 1.15.2-1.5.1.0
Cyclic 1.15.2-0.7.0

Actions: install Cyclic alongside Minestuck, climb the echeladder enough that you have some amount of extra HP, then die and respawn

Result: Max HP has increased by whatever amount extra you're supposed to have, at zombiechum this is 4hp or 2 hearts every death. dying again continues to further increase max HP by this amount, and the change is not effected by relogging or restarting minecraft.

Expected Behaviour: Max HP remains the same before and after death.

The fault is with cyclic's handling of max health modifiers in that version, so that's where changes has to happen to solve this.