You need to update AlchemistNPC to v8.6, tModloader to 0.11.1 and Calamity mod to 184.108.40.206 to make all things to work.This mod is buggy for me combined with the calamity mod. Alchemist doesnt sell anything but the healing potions and once i switch to calamity shop with brewer i cant switch back, its also missing some potions im pretty sure (had potions pre-hardmode that she doesnt have anymore in hardmode). The only way i can get back to the regular potions is reloading the whole game (exiting and re-entering a world doesnt fix it).
AlchemistNPC v8.6 will appear when you update tModloader. Link to it was in update post at previous page.
From what I learned, if versions are not as above, shops may be messed or even go crash and legendary weapons may not work.