SharePoint 2010. We have an infopath form that's basically a trouble ticket. Several (but not all) of the fields are promoted, and appear in the main/default view as columns. These columns can be sorted, and views can be created using filter/sort/group on these columns.
However, when trying to use the Metadata Navigation, the Infopath columns are "grayed out" and can't be added to either the "navigation hierarchies" nor the "Key Filters" lists, and thus there is no metadata navigation available in the library view.
I've searched Technet and MSDN, as well as SE/SO and consulted our SharePoint tech support team, but I haven't found out why Infopath promoted fields are not treated as "normal" columns.
Any suggestions on where to look, or what to try, are welcome.