Hello!
They still want to intervene because they are still guaranteeing the entirety of the Americas from invaders and is more of a design-issue rather than a bug.

If Nation A is in your faction and currently guarantees Nation B, and you declare war on Nation B, Nation B should be able to call Nation A. Though in this case Nation A shouldn't heed the call (not 100% if they mechanically _can_ heed the call since you're in the same faction) and the guarantee should go away after Nation A doesn't answer the call.
I'm still intrigued as to what happens in your game, in my game the guarantee disappeared at least.