I’ve found a bug which may let players stack HP on units indefinitely.
Steps to reproduce:
Give a hero 475 base HP.
Give that hero 10% bonus HP through whatever means you prefer (I used a research upgrade).
Pick up any item (an item that disappears and gives you stats/resources when picked up, I used gold coins) OR learn a hero skill
If done right, your max hp should have been increased by 1. I’m assuming this works with other numbers as well.
This is also not the only bug relevant to HP bonuses, there is another one where going over 100% bonus HP on a unit with lightning orb or spellbook causes it to immediately die upon creation. I can provide test maps for either one of these issues if needed.