This is exactly what’s happening! The branch has moved, but you still have an older commit open in the editor. As you have experienced, if you now try to edit that app, you’ll get told you have to reset to the latest position of that branch before you can make new changes on that branch. So it’s always safe, but occasionally tells you that you need to refresh.
Your actions (switch branch, then switch back) were perfectly valid – although double-clicking on the new commit should also do it.
Eventually, what we’d like to do is when someone else updates a branch underneath you (whether by SSH or just in another session), the editor pops that dialog immediately rather than waiting for you to edit something. That way, both what happened and what choices you now have are clear. But that is one of the many features that lost out in the race between “make it better” and “we have something great now, we need to stop polishing and let people start using it!”