In the BoA2 '75 campaign vs AI I'm playing to test 1.06, I have observed the AI deposit Rochambeau's force in splendid (if short-lived) isolation at Savannah rather than join forces with Washington.
As with the British evacuation of Boston, the AI could really benefit from helper events to get French & Spanish troops from Europe to America. The AI clearly can't handle these sorts of amphibious operation. It would be better if the forces were "teleported" by event to a friendly-controlled port. If possible, the events could be structured with parameters: Spanish troops to Spanish-controlled ports, for instance, or Rochambeau and other French contingents to the American-controlled nearest the Continental Army.