It looks like the fix discussed in
https://www.axisandallies.org/forums/topic/37967/triplea-posting-to-forum-currently-does-not-work
will resolve your issue, too.
In the Triple A version the AI controlling the US always takes the neutral (?) countries in south america. In my opinion only Brazil is pro allies and the rest is strict neutral. Is there a rule that says that the US can turn the south american countries into pro allies?
This is an AI-related bug.
There is - indeed - no such rule.