Honestly my opinion on what is going on? Alternate story from TLK II and there would a be a good few reasons why
First off while TLKII is a great movie the story behind it, if you really think about it, is kinda brutal.. a banished Queen forcibly trains a cub to get blood thirsty revenge and would even kill her own kin. Society has changed alot since these movies came out, by alot of peoples standards that story Arc would been see as too harsh for children so they came up with a different one. One where Simba and Nala had a second child (Kion) and there are no outlanders except there hyenas, and yes Kiara has a personality change, Might also have something to do with her two friends as well that were shown in ROTR
The second point, I honestly think is story. I admit I do not have a sibling but I know people that do..alot of people that do and ..come on siblings fight and bicker all the time, the fact that Kiara even said 'I have my life figured out, what are you going to do' does start a line of sibling conflict. Maybe Kiara gets jealous of Kion, or perhaps Kions steps over the line and Kiara has to be the one to talk him down to prevent another Scar scenario. Either way it would show sibling rivalry and then handle it showing part of the Circle of Life, dealing with your family especially siblings
So yeah while Kiara is different, don't view it as they are saying TLK II didnt happen, or try to mash up storyline cause it simply does not fit. TLG has only begun and I am sure if we all just have abit of an open mind we will actually come to love the new Kiara as well as the old one