So I’ve picked up some mutagen in a basement, and drank it. The game said: “You’ve gained Head Bumps Mutation!” and “You’ve gained Mutation!” When I checked my Traits, I could see the Head Bumps mutation and the others I’ve picked up on the start. Later while battling zombies I got a “You whap the zombie with your tail!” message.
…Best guess is something’s wrong with the “Thick Tail” mutation.