I have sandboxed Google Play Services/Store installed and they have network permission. I installed Google Maps from Play store, and signed out (only from the Maps app, I stay signed in in the Play Store).
After it working for a while, the maps layer stopped displaying anything (maps layer is the default layer as opposed to, for example, the satellite layer). The satellite layer and all other functionality continued to work normally. When clearing all app data it starts working again, but then stops working again after some time. If instead I sign in and switch to incognito mode the map layer works fine.
Also other apps that integrate Google Maps, such as booking.com, mimick what's happening in the Google Maps app. If the Google Maps app is in a state in which the maps layer is not working, it doesn't work in other apps either and vice-versa.
Someone I know is experiencing exactly the same, but I set up their phone, so it might be that it's an obscure option that I set to the same as on my phone.
As work around I can use the incognito mode, so this isn't a big problem. Still wondering though if anyone else here is experiencing this or if anyone has suggestions on why this could be happening or how to debug it further.