I faced this problem with Hugo v0.25.1, but it gone away.
It’s a pity that I did not catch exactly what behavior caused this (I just explored HUGO and I am in first dirty&rapid learning cycle ;-), but I got feeling that there could be two reasons:
when I changed menus in config.toml, than if I use hugo server, than it is better to restart it
another is that I had some problem in the same config.toml
OK great - I also have multilingual menus in config. If you tell me that it worked for you, I’ll keep on trying. I also restart hugo most of the time when I do such changes, but it didn’t resolve my issue so far.
It was a bit confusing when using menus, because they are relative to current page or with “…/” to web/server root while slug to language root, f.ex. http://wwwroot/lv/
And watch out - if you not use absolute urls with "…/section/page your menus will become like …/section/page/section/page when you click in you page.
I hope I had not mess the terms in English an it helps.
Your case is different: you’re changing a slug to an article inside the ‘page’ section. @pascal and I are trying to localize the slug for the section itself (in my case site.com/news/ for English and site.ru/новости/ for Russian), so all menus and other links/internal variables will work correctly.
Looks like most of the frontmatter variables from _index* pages are treated correctly, but not a ‘slug’ or ‘url’. And I also saw an issue that ‘type’ and ‘layout’ are also not supported yet for section’s _index.md files.