Damage Mod Negation On Prime Laser Rifle (Sentinel Weapon)

[Master-Ouroboros]

So I'm forma'ing my sentinel weapons, starting off with Primed Laser Rifle. I currently have Heavy Calibur, Serration, Piercing Caliber & Piercing Hit on it. If I try to add any elemental mods to it, the damage lowers, and my accuracy raises, as if Heavy Calibur was being negated. Is there a damage cap on sentinel weapons, or is this a bug?

[NinjaZeku]

Sentinel weapon modding has been screwed up ever since 18.5, still no fix :(