If it has no performance drawbacks, this bug could be a great feature, pretty much rendering the Mask Modifier unnecessary.

Made an add-on that does the toggling, aptly named: Toggle Normal Maps
https://github.com/theoldben/ToggleNormalMaps

I appended the object into a fresh file. Everything works as it should. I have had 2.8 behave weirdly with the old files before. One time, the file simply had no Render Tab until I appended the scene into a new file.

As a temporary workaround, we use a script to mute the normal nodes, which gives regular viewport performance and another to un-mute the nodes later for rendering:

Just has a default cube, but the normals are long.

Yes.

This also happens when exporting the file. I can be undone, though.
Here is what I got from the Debug Panel