Well, most likely the author helping with the anime production. When season 1 was still in production, Frieren undergo into hiatus because the author was involved with the anime production. There is a reason why there are some new nuances in anime adaptation
If you re-read chapters when they are meeting Macht, yeah they fought 6 centuries ago. And also, one of the reason she stayed in the forest is probably she got PTSD after fighting Macht. Having your arm turned to gold for 1 century is no easy thing.
Nope it is still in character for Frieren. It is a win-win scenario for Frieren, not only she get random spells she wanted, she could honor Himmel's memories in her own way.