a malicious magical being is siphoning magical energies from the brewing of potions, but does not distinguish between witches and alchemists making real potions, and the brewing of beer; humanoid creatures are just as strange and eldritch to this being as it is to humanoids, and the processes seem similar enough. alternatively, the brewing of beer is actually very slightly magical, and beer is brewed in such higher quantities than potions that even the very small proportion of magic it can take from that is still more net energy than it can get from actual potions. this disturbance to the brewing process is making many batches of beer taste foul, and the brewers cannot sell them; supply is low and ingredients are being wasted, driving prices up. potions are also being affected, becoming inert or harmful instead of the intended effect, but this has not been noticed by the general public because they hardly ever interact with magical potions. also because people who make potions are trying to keep a lid on the situation to avoid suspicion from the public who are already wary and distrustful of them. (not familiar with the setting you mentioned, so maybe some of these things don't quite fit, but could be a jumping-off point)