This is not documented ingame in ability description. I talked with Tal about this months ago, yet he said he talked with you, you checked the code and that you said it works as it should... so :)) what's going on.
Honestly I think that defend is badly recoded martyr. I think it was used as template. Still regardless of if that damage should be there or not, I honestly think that even if it's there, then it shouldn't be susceptible to Defense stat, because as such it defeats the purpose of ability (which is now used at lower MP levels to easily give heat to other player rather than to boost def). If there should be this damage applied then it shouldn't be affected by defense stat, since Defend ability is power based.
As far as I remember Marty does damage to itself but isn't affected by negative defense, so it only makes sense to do the same for defend, or rework scaling for that damage it does.
Currently as it is, it's hard to say what it really should be doing, document sources don't mention this behavior at all.