so first things first: it's highly unlikely that this is a bug. Demon Lord has a pretty low base defense to begin with to the point that ignore defense won't greatly increase damage, at least not to a degree where it would balance out the normal variance we see in clan boss runs. 

as well, it's tough to say without seeing what kind of team you're running, etc. as to what normal variance would look like. on top of that, there could be other explanations such as changing speeds, gear, masteries, and a plethora of other variables in your champs/comps.

keep an eye on it if you'd like but it's doubtful there's a bug there. regardless, you'd need to track teams, runs, etc. and have anything before I can send it into support. 

Thank you for responding.  I have used the same team, speeds, masteries and speed tune for at least six months.  Unfortunately ignore defence is now at level 10.  I can no longer raise it to show the difference.