Faster Charging Always Failed, Why?

Have you tried different cables and chargers? If you have a friendly electronics store nearby they might let you try a charger and cable they have for testing.

QC depends on communication between the charger and the phone to ramp up to QC if it doesn’t happen or fails it will default to safe USB standard charging, also check for dust/debris in the chargee port if you have not already.

If this does not resolve it, get it repaired under warranty or open it up to do a visual inspection and likely replace the USB-c daughterboard, or see if the port has become loose and have an intermittent connection to the solder points and fix that.