There appears to be an issue with Ability Capsules. Even though they should not be able to, it is possible to change from a hidden ability to a normal ability, but only if it has one normal ability. Example: It can switch Spiritomb's ability from Noctem to Pressure, but it can't switch Milotic's ability from Cute Charm to Marvel Scale or Competitive.
Note: I did not find this bug. This was tested by someone by the username Borrowedsense, however I could not find a report on this bug.