I'd say if a chapter doesn't advance the plot but builds the characters/world in a way that's relevant to the main theme of the story, it actually counts as contributing to the plot
Otherwise if the scenes build character but in a way that's completely irrelevant to the character's arc, I'd say it's still better relegated to side content (or a sequel that expands further on that aspect of the character) - for the same reason you wouldn't include every aspect of your worldbuilding that isn't relevant to the plot 
Like I have a character in my comic that I ended up writing a lot about, but including those scenes in the comic, even if they most certainly build character, would weaken the overall story imo. bitwam is just not the right story for those scenes. And that's why I'm planning to make a sequel centering this character, because those scenes ended up implying a whole 'nother character arc which is better off standing on its own rather than clashing with the main arc of bitwam XD