There is definitely something suspect going on. I will do a more scientific test this weekend but I have experienced the following behavior. Multiple devices in the family, and also have access to multiple brands of chargers. I did not record times and numbers at first obviously as I wasn’t expecting an issue, but I noticed our 10.5” iPad Pro charging unusually slow on an apple usb c to lighting connected to an anker atom pd1 30w charger.
Pulled out the kill-a-watt and put it in the wall with the setup listed above with the iPad at 78 percent battery. Gave it about 20 seconds to settle in and it was only charging at 8 watts. Replaced the anker power delivery charger with an apple 29 watt MacBook charger and charging settled at 13.5 watts. Switched between chargers and observed the same rates, as well as after a hard reset.
Used my 11” iPad Pro which had battery at 33 percent with an anker usb c data and charging cable, and switched between Apple and anker chargers mentioned above. Apple one pulled 29 watts and anker pulled 30.2 watts.
Haven’t tested my X or sisters XR with either chargers yet but plan to. The way I actually noticed the anker charger working more slowly was it did not warm up like it usually does. Is it possible apple whether intentionally or not borked charging with non apple fast chargers?