Looks like it was too quick! The release notes stated:
Most important, when running the USA DLC game mode, there was a bug which caused towns not to be completely American but mixed (all buildings, European and American, were enabled as a consequence of the bug).
This appeared to fixed when I started a new game today. Sadly the problem has returned when I re-started the game this afternoon. The map originally generated correctly as an ‘American only’ game, it now has both types of streets when cities grow, and also in the ‘streets’ menu.
Looks like they need to re-visit this!
