I am playing version 7.1.1 on Ragnamod VI. When I clicked the Plague Doctor with a Plague Tome, he transformed into the black death and spawned two plague rats, as I am assuming he should. Then he just stood there waving his arms up and down, and I couldn't attack him. I was able to mine the blocks underneath him, but he just floated there. I tried killing other mobs and was unable to do so. The One Probe hud listed the mobs as "waiting for server." I tried to save and quit, but I got stuck on the "saving" screen. Log from "Dance Macabre" achievement. https://pastebin.com/z9PguCiA

Update: I have re-loaded the world (just normally) and the world had reset to just before I used the Plague Tome. When I attempted the transformation again, the issue did not reoccur (I won, yay!). Although I don't know if this is related, I have the plague status effect listed but I'm not taking damage and I don't have plagued hearts.