I’d hazard a guess that you’ve done something funny with the Music Frame Chains on the Page Templates.
In your first screenshot I can see that your First Page Template’s Music Frame is set to use the MI (or possibly ML) Frame Chain. This needs to be set to whatever is the first “M” showing in the dropdown - probably MA:
By default the left and right variants of the Page Template have linked frames, so updating the Music Frame Chain on the left side should automatically fix the right side. However, given you’ve already stumbled into an unusual place, it’s possible that the frames are no longer linked, so having updated the left side you should also check the right side.
You should then go to the Default Page Template and apply the same Music Frame Chain there, on both left and right variants.