I found the answer about the issue on my iPhone 7 with weak and unreliable GPS!!! It was because the iPhone "Case"!
I was testing/driving/walking using the iPhone for a week and it was still having bad GPS status. I was also almost giving up. I think I would report to Apple support some days after I have tested thoroughly. Somehow, I just wanted to change the iPhone 7 case with the simple, plastic shell. (The problem one has some kind of aluminium, metal-like coating on the case to make it illuminated) Once case changed, I still tested it via car driving, and I was amazed that somehow Google maps/Apple map performance were so much improved and accurate... I wonder.....hmmm... oh geezzz, I changed the case!!... Of course, I wanna prove the assumption then, I switched back the case, and bingo! BAD GPS!!!
Turn out, all the bad GPS signal issue is from the iPhone case that made with some metal coating, or aluminium, chromium coating... So folks, just a simple & easy check if this also applies to your phone!, I also do not know if this unique case (but stupid) could have happened to you!