Damage drop off in Coliseum

In Coliseum I’ve found I can’t get my LV9 like past 51% damage against the bosses. My damage drops to nothing suddenly and I lose. It feels like there’s a mechanic adding a hard cap to progress and I’ve seen people mention this.

So my question is this - is it stated anywhere that this is how it works or am I just smacking into a bug. 

My thoughts are if this is a mechanic then it’s the single stupidest thing this game has ever added.