Hi,
Using a USB-C power meter you can measure what kind of voltage and amperage is negotiated between charger and laptop. Most online troubleshooting guides will start from there. It sounds like one of the power converting IC's or usb-c communicating IC's is not doing it's job. But without opening the laptop and checking where the voltage gets pulled low or is shut off by some component, it's hard to tell. This channel has a lot of MacBook troubleshooting/fixing:, maybe your problem is reproduced on there: https://www.youtube.com/channel/UCl2mFZoRqjw_ELax4Yisf6w