I see this is a fairly common problem - judging from the number of posts regarding it - that has not been solved so far. It appears - at least in my case - to be a problem on the user side, isolated to a particular device. My iMac, iPhone and iPad show all maps properly, only the Mac Mini shows a grid instead.
Is this something Apple doesn't like to talk about; and even less working on?
Mac mini, macOS 10.14
In the end it was really simple to correct: In the user library select the cache folder and throw it into the trash. Empty the trash, restart Maps and voilá, the maps show again.
Nothing fixes a Mac like throwing the Cache into the trash!
Starting in Safe Mode also clears the caches.
It's supposed to. But it either doesn't do it to where it's effective, or it doesn't do it at all.
I had been booting up in Safe Mode at least a dozen times without results. So I finally decided to dump that folder completely. After all, all you lose is some convenience; that thing rebuilds itself over time.
FWIW, to my knowledge starting in Safe Mode only clears certain caches.
Apple Maps shows only a grid on Mac Mini.
