Which defender are you talking about Aegon's fight against? It's been so many months since I completed my first necro run that I can't remember. and the few times I used Aegon it was quite... stupid (probably because I couldn't keep calm when I was overwhelmed lol) Sorry for not satisfying your experience!
if a sp3 is thrown and you have lets say 20% its supposed to instakill but the heimdalll synergie activates the inderstructable at the 20% mark before the sp3. so when it ends it dosent heal its just you took no damage and the 1% is kind of like a visual glitch