I'm having the same problem too (when plugging in the type C to lightning cable). Looking through your comments, I tried the following:
- Close all standby apps
- Go in low power mode
- Airplane mode (FULL ON with no wifi and no bluetooth on)
Still unable to support fast charging or constant charging to begin with. (It charges and then a few seconds to a few minutes, it will keep restarting itself).
One thing I will note is that I was using a 3rd party 4 ports charger that has one QC 3.0 USB Type C with the rest regular USB ports. This problem does persist when I plug in a little too many devices to charge that draws too much power at the same time.
This ever happens when its from USB type A to lightning no matter how many I plug into the power plug.
Lastly, I also bought a much more breaded USB type C to lightning as the final straw to see if its the culprit.
So I believe it might be the iOS issue in the very long run.
I'm on iOS 14.4.1 as of writing of this feedback!
Looking forward to the community here if there was anything else tried.