Bug introduced in V9 and fixed in V11
Same story as in NotebookImport ignoring multiple style cells,
CellPrint @ Cell["test", "Section", "Program"]
Cells[CellStyle -> #] & /@ {"Section", "Program"}
This cell was only found by "Section"
selector. I'd expect "Program"
to work too.
And again, multiple style cells are officially supported so I consider it bug.