Order a factory adjacent to a mass pile, hover mouse over the pile: a mex is selected for you; hover mouse over the factory: mex ghost is moving with the mouse over the factory, and you cannot select the factory.
Automatic Extractor Selection bug
This is an item in the options menu called automatic mex selection or something similar, it's a new option that defaults to being on. You just have to remember to turn it off.
I'm thinking of setting that feature to 'off' for default, as it causes a lot of strange behavior / bugs.
A work of art is never finished, merely abandoned
Also you should be able to select the factory if you click twice.
It's just that the game autoselects the build mex option for you if you hover over a mex spot.
You can just unselect it by left clicking anywhere that's not a mex spot.
Might be more interesting if it could also unselect the mex again, if you move away from the spot. (with some margin, to avoid flickering)