This is the issue: apparently the only way to successfully link a nav bar menu item to a section in a multi-page site is to put a full link (something like https://www.website.com/page#section). The problem is that this will reload the whole page even when the link is invoked on the same page. Webflow should know whether the target section is on the page already being displayed: if the section is on the same page, it should just scroll to the section. If the section is is a different page it should load that page and then scroll to the section. Right now it always reloads the whole page, even if it already being displayed.
I know you can get around this by creating different nav bars for different pages, but this is a messy solution (multiple nav bars in the same project is very problematic, and more work)
I still think it would be nice that when linking a navbar menu item to a section in a multipage site, Webflow would automatically adjust the code so that the link is not dead when invoked from a page different from the one where the section is. Minor change but it might help other people in the future (and avoid dead links).