This is probably a long post - but I would like others to know, that may be experiencing similar issues.
First of all I have an S8 (purchased new) - kept with the same carrier all its life. Unit is basically stock - e.g. no modifications, not rooted.
I have had smartphones since 2005, and Android since 2011 (I suspect I have had Android earlier - but not trying for an ego trip - just point out I am not a newbie, and after 36 years in IT - at the coal face - no dummy in the diagnostics dept either)
Besides normal email, it has been used with Android Auto, combined with Google Maps or Waze for the last two years....the occasional issue mostly resolved by forum posts such as clearing the cache, checking certain settings for location etc.
Typically I have a 30km drive to work and back again in open terrain, so I have been able to run a series of tests, changing one thing each night, and testing for 60Km the next day.
However 4 weeks ago (appeared to be after the September 2019 security update), both Google Maps and Waze became unusuable due to the constant loss of GPS signal...it would appear to get a lock, then lose it 5 seconds later. Let it run like this for a week - defintely no improvements...over this time, powered down the phone, read the forums on others that are having the issue.
So for the next three weeks, I ran some tests as time allowed.
1) Many of the posts on various forums basically said the GPS has gone faulty. Confirmed this was not the case using ChartCross GPS test. Tested it at multiple locations, including during the drives two and from work. No noticable loss of GPS....in most cases showing 26 sateliites most of the time, using 25 of them (listing the peak figures, but they did not go much lower than that.
a) Android Auto App disabled or exited
b) no external power - just the charged battery
2) Now I had confirmed that the GPS was not faulty, the tests were done on test 1, were done with no external power applied.
a) Android Auto App disabled or exited
b) external power provided by car via USB
Summary, at this point this confirmed the GPS was not faulty - working under battery and power
3) Next set of tests were run over several days and a series of changes - working through the posts, just about trying every idea, which included to the best of my memory :
a) update AGPS data for faster fix time (this was not the issue though - but did it anyhow)
b) Performing program cache clears on all applications that use the GPS
d) Checking Power settings - running the phone unlocked / no screen timeout
c) Checking and rechecking every switch known to forums everywhere, which included the power settings, including setting them to high, disabling the GPS related apps from Optimisation. Everything...still the same issues
4) Now I removed Android Auto from the phone all together
I run over several days - Google Maps and Waze powered by the USB in the car, and left just displaying on the phone.
Google Maps and WAZE working perfectly - no noticable loss of GPS on the maps
a) Powered by car
b) No Android Auto installed
c) using Google Maps and Waze
5) Now Reinstalled Android Auto ( before starting it - cleared Cache Data and Program Data)
Same loss of GPS signal using either of the Waze or Google Maps via Android Auto
6) Now tried running Android Auto with Waze and Google Maps via Android Auto but not connected via USB (not displaying on head unit)
No loss of GPS signal on either app - ran like this for several days just to make sure.
7) for the next few days focused on Android Auto permissions and power settings
Absolutely no improvement - went back and run several tests again to confirm I am still working with the same assumptions - confirmed GPS working - rechecked a lot of my tests
8) Finally today, I bit the bullet......
Performed a Factory Reset on the phone - complete
No use of Backups and Restores - I wanted the phone as clean as I could.
The only thing I allowed myself was using the same google login (yes I know some google data will come back down to the phone and It will reinstall some of the Google related apps + carrier base apps)
Final Result : After running through all the wizards for the phone, the Waze install and setup, the various configurations that need to be done to the phone, including bluetooth pairing and Android Auto pairing
Run Waze as my test - instantly with 1 min of driving the GPS locked and after about 20km with a few stops and removing the phone from the car each time, the GPS apps were working the way that they did before, with faster GPS locks each time I got in the car.....
What was the Issue:
I strongly suspect some sort of permissions issue or power settings issue. I wish I could spend longer looking at it, but I suspect it will be a setting that may not be a visible one, the result of applications and security upgrades clashing or new settings being set as default which made an impact.
Anyhow sorry for the long rant...but I hope it gives some ideas to others, even on thinking of ideas to perform certain diagnostics to help themselves get to the root of the issue. Even if you are thinking of a factory reset, to provide some hope that it will resolve the issue (just remember to limit what you restore as you might just bring the issue right back.