Fountain Pen Help

[ddeuel]

Hi I write primarily with a pilot vanishing point and use only Pilot Iroshizuku inks, I own a blue, black, red, and as of last week a golden brown. This most recent ink, the golden brown, looks beautiful at times, but for the first couple hours of my day it writes almost black and starts to slowly lighten until it is fully golden by the end of the day. I am fairly inexperienced with fountain pens and would appreciate the community's help on whether this is an issue with my pen or the ink, or just how this ink functions. I've never had this issue with any of my other inks. Also I attached a picture of the difference in the color from the beginning to the end of the day.