Hi,
I’ve just started a new document using the Solo piano template. I haven’t made any changes to the page, but I did go to the Master page and made some changes there. These haven’t flowed through. There’s no red triangle anywhere.
Glad that fixed it. If you draw a new frame, or resize an existing frame, it won’t mirror automatically. The only time it does mirror is when you change the contents of a frame that exists on both sides.