I just received my SHARKK 3.0 wall charger as recommended in this thread. Unfortunately, my G5 presents the same symptom - it says "fast charging" for a couple of seconds and then reverts to standard charging. I tested with multiple USB C cords of acceptable quality, including the stock LG cord. We need some more results from G5 users and 3rd party 3.0 chargers. Maybe some are not affected by this issue? I tested with another G5 in a Verizon store, and it also presented the same issue.
That definitely isn't a good thing, both myself and my girlfriend have the G5 and we use the same exact chargers and she also doesn't have the issues you're talking about either. You say you've tried the chargers at the Verizon store and their phones did the same thing? Dare I say that maybe Verizon did something with ability to uses quick charge 3.0? Maybe try to use the chargers at a different carriers store? I can confirm that with tmobile my phone will say quick charging with the stock charger, Samsungs adaptive chargers and with my aftermarket chargers, aftermarket being incredibly fast.