So basically, you're complaining that a status effect which converts healing to damage works as intended.
There's two ways to look at it:
1. It's clearly not intended for you to be able to chunk nearly a third of a boss character HP with a basic "combo" like that, which requires no investment beyond the ability to have those two spells on your hotbar.
2. Touch of Decay should, from an immersion and gameplay standpoint, only invert the explicitly magical forms of healing.
First Aid is not a "spell" it's a physical action that for whatever reason we're able to perform at range(probably because "fuck reason" seems to be a mantra for the designers).
Unless the spell sets bandages on fire, bandages should still have the intended effect of acting as a treatment to the injury.
