Unable to Filter Contacts with DisplayLabel

I am trying to filter a ContactModel using the contactdetail DisplayLabel
as below: However no contacts are returned.

//This one refuses to work!!!
filter: DetailFilter

{ detail: ContactDetail.DisplayLabel field: DisplayLabel.Label value: backEnd.searchKey matchFlags: Filter.MatchContains }

I can filter using the contactdetail Name, as below:

//This one works
filter: DetailFilter

{ detail: ContactDetail.Name field: Name.FirstName value: backEnd.searchKey matchFlags: Filter.MatchStartsWith }

The contacts do have DisplayLabels, and I am able to sort using DisplayLabel

SortOrder

{ detail: ContactDetail.DisplayLabel field: DisplayLabel.Label direction:Qt.AscendingOrder blankPolicy: SortOrder.BlanksFirst }

As this issue was encountered on QtMobility, I tried to reproduce it on Qt Contacts 5.0 / Sailfish OS. However on that platform I am not able to filter at all (yet), and have opened a separate issue: https://bugreports.qt-project.org/browse/QTBUG-35276