Since fix CMS-8689, the widget SelectReferenceTable returned contents of all types.
The value of "property-contentTypeOrMixin-eq" was ignored on server side.
Indeed "contentTypeOrMixin" is not part of the search model criteria.

I solved this widget using the id of the "dynamic" search ui model for reference table, for which the content types is set in model (see CMS-8703)

But I think there is still a issue for the SelectContent widget. The default "select-content.xml" search model has no criteria "contentTypeOrMixin".
Moreover, the used model is overridable in this widget, so we cannot known how the model is defined.