it was when you equipped one hero directly with item from another heros backpack that it came into effect.
I.e.:
Hero 1 has item X in backpack
Hero 1 puts item X on ground
switch to Hero 2 using the arrows in the popupscreen
Hero 2 picks up the item from the ground and equips it directly.
This caused the item bonus to be corrupted. It is now fixed both in version 0.8 and version 0.9.
If you have equipped a hero this way earlier than now you probably have this problem. Let me know and I can fix your hero if the game is important
