That explains it. It's not a glitch. This is normal behaviour for the V5.
Your normal adult characters can change into various abnormal special characters if their care has been extreme. For example, if you consistently over-feed your V5 characters, they can turn into the Large family characters. In your case, you consistently kept the number of filled happy hearts low. After your normal characters have changed in to special characters, you can change them back by doing some specific action. In the case of the Large family, you have to let their hungry hearts empty then feed them a balance biscuit. In the case of the Gozarutchi (Ninja) family, you have to feed them a snack.