I think that if those colonies rebel due to low public order for several consecutive turns, they turn into something else. I've seen Austria re-emerge twice and Wurttemberg once in this campaign after being eliminated, because the home region went into rebellion against the new overlord. Mexico just popped up last night as a nation even though Spain still exists, so I assume it too rebelled. Since the colonies rebel individually, I expect that's why the Commonwealth of New England was the only one involved. Now, if they lose New York or Pennsylvania, perhaps they'd join the US as well.
It should be interesting to see in your game if the Thirteen (now Twelve) Colonies go to war with the new single-province US.
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