The way I understood it was that she did not loose her ability to use magic, but rather decided not to use it any more after what happened when she had to draw power from fire.

Spoiler

She also used magic to escape Bonhart/Skellen/Rience after Kenna’s attempt to read her mind unintentionally gave her a bit of magical power.

Even if it would be a bit disappointing, I think an explanation as simple as “she grew older, and had a lot more time to deal with the trauma of what happened in the desert” would be sufficient to explain why she is OK with using magic again.