How to fix in-app (google) maps after Google Play services update broke map functionality?
Most surprisingly, after reverting and/or updating some of the relevant Google apps, some settings got restored to some previous values. Those may also include permission settings and in my case, the Data Usage settings. 

I solved this by first ensuring that the following 2 system apps were up-to-date: 


Google Services Framework *
(com.google.android.gsf)
Google Play services (com.google.android.gms)


Note that GSF is normally never updated and remains fixed for your AOS API version.

Next, I went to check the permissions of those apps. That's when I found that Mobile data had been mysteriously disabled under data usage. Check by navigating to:

Settings &gt; Apps &gt; Google Services Framework &gt; Data usage &gt; Mobile data: "allowed" 

Check both Mobile data and Wi-Fi tabs, and for both apps above. That fixed it! 

For completeness, also check: 
Settings &gt; Data traffic management &gt; Networked apps and check that data is available on both the tabs Installed apps and System apps. 



But since you're on it, you might as well also update:


Android System WebView (com.google.android.webview)
Google Play Store (com.android.vending)


If you still have map problems after this, then (a) make sure you're using compatible binaries (APK) for your specific device (API, DPI, arm version etc) and (b) that the permissions for all of these apps are as allowing as possible. Also, the device clock need to be correct...