The person in the comments answered it correctly to my surprise.

i18next warns about incompatibility with versions of TypeScript below 5.0.0, and as my version in frontend was 4.9.5, it would not let me use this feature with namespaces.

The problem is that LanguageDetector and I18NextHttpBackend only fully work with TypeScript >5.0.0, and their usage with older versions will result in surprising errors in some cases.

TLDR: Upgrade TypeScript to >5.0.0