In a single metadata browse index the sort options always show "No Entries in the index" when trying to sort the browse index of a single metadata value,
e.g. browse by subject "keyword3"
Gives 4 items the titles starting with "T", but the selection of sort by title "t" gives no results:
It does not work with the xmlui either but at the moment the xmlui neglects the selected value see DS-1575 you can only reproduce this by manipulating the URL directly.