Views sort on indexed string field causes no results

I upgraded my site from Search API 1.3 to 1.10 (and subsequently to the latest dev version). All of my views based on search indexes stopped returning results. I tracked it down to the sort criteria. If the view contains a sort based on an indexed string field, no results are returned to the view. Integer fields can be added as sort criteria without a problem. I am using Solr. This happens on both node based and taxonomy term based indexes. No errors are logged while indexing items, nor creating or displaying the view.