Years ago, here in the USA, I'd only heard the word "proper" to strongly imply that something met unusually high and unnecessary standards that someone else had set, or imposed on others, according to their own beliefs;
often in another place and time, that were overly and exaggerated as in "act and dress, prim and proper"
The unusually high and narrow standard meant that a person could not add their personal expressive touches, and needed to conform to very strict rules of assumed politeness, to an overboard extreme extent,
that might even be contrived and not honest or usual,
rather than to be basically polite, while remaining true to yourself, whatever your true style or habit or comfort would have directed you, yourself.
Am I the only one ?
who had originally thought it was only defined that way, until I heard others from UK especially, using it for a more general and accepting way, and not nearly as "judgmental" of others, as how I'd heard it used ?