Broken: 2.7x, 2.8x
Worked: 2.7x, 2.8x with UV Sync On
Short description of error
Changing to Face Mode in the UV editor doesn't deselect vertex/edge that don't make a face if UVsync is turned off. same if you have a vertex selected and change to Edge Mode.
Furthermore, changing to UV Island mode should deselect vertex/edges/faces that don't make an island.
Exact steps for others to reproduce the error
go to UV editor tab
select a UV vertex in the UV editor
change to edge/face/island modes, the vertex will still be selected
now select UV edge
change to face/island mode, the edge will still be selected