So, those might be oversights for all I know. Well, at least they make minimal sense to me personally.
“Plaguebearer’s” rare prefix - clearly acid oriented, but lacks one thing common to other damage type related prefixes: physical → acid conversion. Intented or not, this prefix is otherwise pretty barebones. Sure, it provides RR, but it’s a small amount compared to Manticore. Or if you can’t take it from constellations, Mark of Dreeg component exists.
Second one is one of the Dreeg’s plagues - Plague of Fatigue. It converts phys and acid to chaos, granted skill deals chaos damage, but it reduces… vitality res? Considering another two dedicated vitality plagues exist, this makes it rather redundant and not that very useful.
So… are those “working as intended” or not, I wonder.