Done that many times, no change.
The "system" obviously recognises the two types of monitor - and it names them correctly.
It just refuses to allow the left monitor to be on the left and the right one to be on the right.
As I said - I had this before, but it "cleared" itself one day when I clicked the dualview application settings in the control panel.
Very odd. Indeed - it's more odd than a very odd thing.