Steps to replicate the issue:

• In a VisualEditor session, try to add a new (non-existent) category to a page, that is named Wikipedia:Projects, using the built-in "Categories" page-Option

What happens?:

• The prompt to create a "New category", is auto-truncating the prefix.
  • E.g. Typing in Project:Demonstration will only let me use Demonstration as the category-name.

What should have happened instead?:

• It should offer the exact category name that I wrote.
  • Perhaps that would be in addition to the prefix-removed category-name, if that's needed?
  • I would guess that we're currently auto-truncating it, in order to solve the problem of users trying to paste in the full page-title including the Category: prefix itself (or localized variants), which probably should still be removed (if easy).

Other information:

• This is a common category-naming convention on Nlwiki. E.g. See the category-tree that begins at https://nl.wikipedia.org/wiki/Categorie:Wikipedia:Projecten - E.g. Categorie:Wikipedia:Projecten_Wikimedia_België
• It seems to only affect prefixes that match namespace-names. E.g. It will not truncate a new category-name like Foo:Bar, but it will change project:foo:bar into Foo:bar.
• Original bug-report at: https://www.mediawiki.org/wiki/Talk:VisualEditor/Suggestion_Mode/Feedback#c-Geertivp-20260823194200-Quiddity_(WMF)-20260822181400