I found an easier way to clearly show the “increases with” operator does not work properly. Try this:
hero health value decreases with 20 hero says "HP = 50 - 20 = 30" hero health value becomes 1 hero says "HP = 1" hero health value increases with 10 hero says "HP = 11"
Just have a closer look to the health bar while executing this. The maximum health of my test hero is 50. After the last line the health bar is full.
The decreasing seems to work properly. I also tried the increasing with just 1 HP. This will restore about 10 HP I would guess from the health bar.