I have several search models in WEB-INF/param/search folder. In each search model, I have a criterion on contentTypes system property. The ContentTypeEnumerator is initialized with the first found search model restrictions (ContentTypeSystemProperty line 308) are reported to the enumerator and the same enumerator is used for all search models.

So a "contentTypes" criterion in search model could be too restricted or too large (depending of cases).

You can reproduce the problem on the main search of ODF, all content types are available but we shouldn't have private content types.