Find a better repair shop because they should have never given your Switch back without fixing the problem.
This is a pretty basic repair.
Some details about your issue :
The Switch has 5V charging with a 5v only phone adapter and 15V fast charging with a PD or original adapter.
It sounds to me that the 5V circuit is fine but the 15V charging circuit inside the Switch is still broken.
The Switch probably tells the charger to send 15v but internally it can't convert it for charging the battery,
The Switch had nothing to check whether the charging process is going well or not.
So slowly you will be draining you battery when connecting it to a PD adapter.
Besides the M92, there is also a BQ24193 chip that handles charging.
They work together to make this slow/fast charging possible.
An extensive write up about how the charging in the Switch is supposed to work can be found here :
https://www.retrosix.wiki/bq24193-battery-charger
There is plenty of information on GBATemp about the M92 and what to measure.
Besides the ICs themselves, also surrounding components like capacitors can be broken.
Hekate has a page of full of info about the battery and charging ic.
This is a screenshot of a broken system so don't mind the numbers.