Different masterpage in one layout only

Maybe I’m missing the obvious, but in the attached project, when the piano part layout is opened, the default master page (for 1st pages) is different from the default master page for all other part layouts.

Interestingly they seem to be “connected” somehow: if you change the height of the text frames of the default master page while the piano layout is open, all other layouts are changed.

There are no page overrides in this project.
Any ideas, what is going on?

Vorlage Song.dorico.zip (407 KB)

You’ve changed the page margins in the Piano layout, and because the frames are “locked” to those margins, they get stretched along with them. The Master Page doesn’t give “absolute” measurements for the frames, only the # of millimeters distance from the margins. That’s why the proportions can change.

Ah, missed that one. Thank you for your help!