I was thinking of her resignation from the Power Rangers set. Made me think, did she really already end up wanting to resign after MMPR started going on a rather bad direction? As far as what was known, she resigned professionally and had a nine episode arc to write her character off but behind, that, she might have really wanted to be written off as soon as possible. So what's my speculation?
I guess Amy Jo Johnson probably wanted to leave the set around season two when Kimberly's character started suffering from her development shrinking. So what happened? Kimberly well her fighting might be sub bar but she could fight one way or another. Her fighting started to improve in the first season but she was later changed to Tommy's damsel in distress which was really getting annoying (and I was just staring at her). Also during "A Friend in Need" she was reduced to such a pathetic role of being sick in bed which really for me, was one of the worst scripts written.
In between, maybe Amy Jo Johnson wanted to leave the set because of the direction the show went through. Okay she did appear in PR Turbo but after that, she won't reprise her role again and I don't think she'll even appear in Super Megaforce. I guess she does still cherish her time in Power Rangers but ended up having mixed feelings on it considering the bad direction the show went into. It was highly possible her resignation later was thrashed too many times because the higher-ups thought they still needed her for ratings. But yeah, she was probably mad that she was probably being misused as merely eye candy.
So maybe in season three, Amy Jo Johnson was happy or more than happy to be out of the cast for good due tot he arrival of Catherine Hillard. Maybe they thought it was time to give her what she wanted, her break from the show but I guess they all had ill feelings that led to the Dear John letter scenario when they couldn't get her back at all.