Looks good, that's some gnarly monkeypatch/mock testing but I think it covers things nicely at the unit level.

I think the main thingto add here is an integration test which demonstrates the behaviour of loading a file with these invalid coordinates. This is not entirely obvious from the loading code which is quite complex, though experiments show that these variables tend to be ignored completely rather than become their own cubes. This is in a way preferable to the current behaviour where nothing would be loaded, though it's worth considering future improvements to this behaviour which retain this information in some form. I think such improvements would be quite complicated though, so we can consider this out of scope for the time being (though it would be worth adding in a comment in the integration test stating that the behaviour tested for could be revised in future).

Just a couple extra details to look at.

Looks good!