Field parameter bug?

I've set up a field parameter to switch the location field around from Region-Country-City.

It works fine and as I expected when I first set it up, however the next day dragging it in as a slicer it's no longer returning the "Region-Country-City" selections but the actual data from those fields, just like I had dragged in the location hierarchy, so I get the whole thing like this:

Could it be related to the fact the fields are in a custom hierarchy? It's just weird it works ok at first...