That's not a bug- if you switch to another character, you lose all progress on a previous character, which is intentional. It's supposed to be a hard decision (if you've put in work to boost a character) to switch to a new character: are they powerful enough to be worth it?
Do you mean from spending them on a new character's banner?
Hmm. That is an issue. I'll look into that!
EDIT: Alright, it should be fixed - the issue was that when setting the UP! character, it also set the charBoost to 0. I don't know why that was, and now the charBoost is only reset when you get a new character.
starting at dataLevel 1.3, you can access boss fights! these are unique, ultra-powerful enemies that have special different powers. defeating a boss gives you extra points! each character will have their own boss fight.
currently supported boss fight characters (you can engage in boss fights with them): Default, Bomberman