This is Magnomagus decision based on his vision so I cannot answer why the specialization changed.

However I assume that the specialization itself becomes useless in H55 because it basically gives the hero what other heroes can get from the rune guild with no specialization. Having in mind that H55 maps have a lot of castles, a hero needs 2 fortress castles to gather all runes. This makes Inga ability weaker because she gets something that other heroes get for free.

Even early game her specialization does not help much because rune casting require resources which the player cannot spare at this stage of the game.