NHF – new heroes for Heroes V 3.1.

I noticed something. With Melchior and Tanna, I've played them several times, and it says they have a 50% chance to learn spells as they level up, but after leveling the heroes to level 25, they didn't learn anything. Or was I just unlucky? If I misunderstood something in the description of their special ability, please explain how it works.