And now you want us to guess which pages work correctly and which not?
BTW, you write above, that everything works fine. So what should we do?
However some of them do work correctly; they just open in the same window. How can I fix that?
Which do not work correctly? What do you want to fix?
I don't see any hyperlink opening up in a new window!
Can you please share a screenshot of your hyperlink window.
Also, check if it opens in a new window while doing a local preview?
Links on this page Geomembranes and membranes for gas protection do not open in a new window.
Links on this page Specification sheets and products information about contaminated land open in a new window.
It is the same on local preview.
They are all set up the same way in Muse, i.e. 'Open the link in a new window or tab'.
One way to do that is to export the site as html and use target="_blank" for all the href tags. See https://www.thesitewizard.com/html-tutorial/open-links-in-new-window-or-tab.shtml for more info.