The ineptitude of the Samsung geniuses, with their "update," makes this appear more like an iPhone, which I didn't buy.
Fast charging, be it cable or wireless, while checked, does NOT work. I spent two hours with a Samsung tech and an hour with AT&T.
The irony is, 24 hours before the update, I was about to buy a Galaxy Watch. I have since changed my mind and will be purchasing another brand, one who prefers to keep their brand in the hands of people because the changes are correct and for the right reasons.
Mine is doing it too, started right after the most recent security update. I've tried Samsung fast chargers, aftermarket Quick Charge 3.0 chargers that I know worked previously, and different cables to rule out bad cables... It's the same slow charging message no matter what.
It says fast charging when I first plug up the phone, and sometimes it'll stay that way but 9 times out of 10 it goes to slow charging after a few seconds.
I'm traveling right now so this is happening at a bad time. When I get home I'll see if it is acting up on wireless charging... I use wireless charging 99% of the time at home.
Even I have faced a similar issue. I have also openned a support case earlier and these guys have stopped responding on this. Try the steps written here, which i have followed, see if any of those work for you.