I just got a message "You have not researched Governor yet, so you can only occupy 1 ports!". I don't remember this being mentioned anywhere previously - not even in the research page for Governor.

I suggest a warning message if Governor is not researched yet and a player is trying to capture more than one port (both if one is already occupied by the player and also if another fleet is only on its way to capture a port) and also an addition to the description of Governor research mentioning that more ports can be occupied after researching it.