Hm, I’m not sure but sounds a bit like the problem with solr search we had.
We migrated about 7.000 articles via api with no language set. But then we set our default language to de_DE. All new created articles then had the language de_DE. We never found any strings from the content of one of those de_DE articles. (Titles we found.) For testing we switched our default language to de. Now articles newly created with language de would be found immediately.
As a workaround we for a short time switched to multilanguage and changed all de_DE articles to de manually and switched multilanguage off.
Maybe you can test it with nl vs. nl_NL?
(We run 13.10.6 Debian.)