The order in which a TC is attached to a lens question...

I'm guessing the extender has internal drive electronics which talk to the camera as soon as it's fitted.
I suppose it's possible for a mis-communication if the lens is fitted last.

That said, I use whatever order is convenient at the time.

There have been folks on the forums, reporting problems when using an extender. Problem went away when the converter was mounted on the lens, then the combination on the camera, instead of the other way around.

I don't remember exactly what the issue was, though.