I have multiple oem rapid chargers, and Anker quick charge bricks, along with multiple Samsung oem USB c cables. It never indicates fast charging.Nope, mine Fast Charges without an issue, however a colleague of mine was having a similar issue on his S8 and we found that his charger brick was at fault.
Do you have Fast Charge turned on in the Battery Settings? Also try test with someone else's Fast Charger to try isolate the issue further.
It gives a estimate of a 1-1:45 to charge, but misses the mark by a lot. A full charge even left alone takes about 3 hours now.
It was always a bit finiky about allowing fast charge, but now it is impossible. I even factory reset it, but it didn't help.
Super annoying.