Layouts are like Viewpoint in that they store the current Zoom factor.
So when you open a Layout it will restore the Zoom factor that was used the last time you worked on that Layout.
If you go through all Layouts and do a Fit in Window on all of them, then save the file and reopen it, you will find that all of them retained the Fit in Window zoom factor and they now show the whole Layout.
But this also means that if you zoomed in on a Layout and leave it, the next time you go back to it, it will still be in that zoomed in state.
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