This shouldn’t happen if you’ve set up your master pages properly: each master page is a pair of pages, left and right, and when you end up with a particular master page on the left or right side of a spread, then the appropriate master page will be chosen. You shouldn’t ever really have a master page that you think of as just a “left” master page, therefore, as either the left or right page will be used according to where in the layout that master page happens to fall.