SORRY FOLKS, I found the problem. I had a function elsewhere which set the mouse input as handled.
func _input( event ):
if event.is_action_pressed("Mouse") and not event.is_echo():
Signals.emit_signal( "tileClick", selectedTile )
get_viewport().set_input_as_handled()
Unfortunately, had forgotten that I added the last line to handle an issue with mouse clicks being duplicated several times. Commenting out the last line fixed everything. I may have to find a way around the mouse duplication issue, but otherwise everything is fine. Remember to scan your code for anything that marks inputs as handled, and put all your input functions in the same place!