Bug with Iron Gut feat from Grim Hollow source book.

My character took the Iron Gut feat and mysteriously, her HP went up by 5 points. There isn't any part of the feat description that mentions an increase to max HP on the D&DBeyond feat description or other sources. Thinking it was tied to her level, I reduced her to level 4 and it continued adding 5 HP. Changing her CON to 18 instead of 20 made the bonus HP 4 instead of 5. I was not choosing the CON ASI included in the feat and when I removed the feat, her HP went down by 5 so I am certain it is an issue with this feat or an interaction it has with another feat, maybe tough.