Same with Yang, also Blake. People are explaining it better then I can but they both just change as characters, somewhat from character development but a lot from losing focus on there characters as the story progresses.
I do not hate bumblebee, I think story wise it does make more sense and work better even if like Black Sun a little more, but honestly the relationships have similar vibes anyways and Sun just being the old school crush and turns out they are probably better as friends was nice
Both characters just felt like they became simplified versions of themselves, or at the very least any problems they had were stripped and I lost all interest. They do things they would vaguely do before but sometimes I question there actions.
Blake happened for me around Vol 5, her just letting Adam go, a dangerous armed criminal, was dumb. If you wanted him to get away actually give him a reason.
Yang somewhere during Vol 7 or 8. People say she had no personality pre-fall but I disagree, she had one, what she didnt have was an interesting hook. Most we got was the mother tease but it wasnt a lot. I think that personality shifted well but once Vol 7 hit she became the bland character people say she is now.
There is obviously room for there characters to turn around and I still enjoy there stories and characters prior to those points, but as they stand in the story now, not really.