Armor Penetration and Armor Breaks

I was recently testing out my 7* unduped Mangog and I noticed that when I got to 3 armor breaks while I had the armor penetration stat focus active, the stat focus essentially shut down and I got the same damage on basic attacks as when I did not have armor penetration. This persisted as I got to 4 armor breaks. I would be getting more damage with 2 armor breaks and armor penetration as opposed to 3 armor breaks and arrmor penetration. I tested this in multiple fights and got the same result. Because of my testing against Heimdall I was able to determine that armor penetration does still have an effect when the opponent is at 0 or negative armor, and even if it were the case it still makes no sense to me that my damage dropped when I got to 3 armor breaks. All things considered I was wondering if there are any issues with the way armor penetration works with armor breaks or an issue with 7* Mangog?