It’s been reported.
Upshot: ghouls can’t be used to increment plague because they die too easily. Probably means that it can’t be soloed, although it can be duoed if you are careful to not allow Necrotic Plague to waste a hop on a ghoul (by killing them immediately, only allowing plague to ping-pong between players.)
I’m sure they’ll fix it someday. AFAIK, they just haven’t yet.