Maybe unrelated, but still one thing to be aware of - the render dialog has a bug since max 2020 (or before, not sure) where it shows whatever viewport was active when the render dialog was opened, even if something else was set and locked before. So when you've set and locked it to render from camera 1 and happen to have camera 2 as the active viewport when you open the dialog, it will render camera 1 but show camera 2 in the rollout.