Heavy Caliber keeps losing fusion cost

[IcarusValkyrie]

I had maxed out heavy caliber mod and unfortunately used the last of my endo to do so and now all of a sudden it lowered its self down by 1? I spend 1.4 million credits and 30k endo for the mod and go no refund for it deranking itself. A week later the mod had lowered its self down again by 1.