If I were to make a suggestion, I would definitely prompt the users to save. The prompt can be subtle, such as changing the color of the save button to something other than grey to help signal that changes haven't been saved. Additionally, I would set the page to automatically scroll down to the save button. It might be different on desktop browser, I am solely speaking about the mobile website.

Attached is a screenshot of what users see when they update their banner. What I am suggesting is making it a tiny bit more dumbie proof.