I've also had this happen. It's definitely some interaction with the Knusperhexe, but interestingly it's not a 100% of the time thing... except against bosses/challenges, where it seems to be much more reliable for some reason. "Fixed" it in my save by building over my Knusperhexe. I wonder if it would be possible to trigger this bug with towers other than Mr Iron?
I filed a ticket for this - Fix healing attack problem with Knusperhexe/Cactus combo (0000296)
Target milestone is 0.9.3!
Thanks for reporting and your investigation :-)