It doesn't work like it should. It should heal only 500 health over 12s. = 125heal per tick 3s.
How it does work now. It scales with Bonus Healing adding extreme amount to the overall healing per tick. For example: with 0 Bonus Damage and Healing it heals for 125, like it should. With 952 bonus healing it now heals for 315 per tick, which makes it overall 1260 total. The more Bonus healing the more healing per tick and overall will increase.
What needs to be done. Remove scaling with bonus heal and bonus spell damage to fix the problem.
Spell it uses: http://www.wowhead.com/spell=38299/hots-on-heals