Based on what you’ve said already it is plausible this is board issue, but I would try a firmware restore just in case. You will need access to another mac that can run Configurator 2. (https://support.apple.com/guide/apple-co...). If that gives an error, that can also help point towards what the issue might be. Keep in mind it wipes the SSD entirely, and it will need to be reinitialized.

Also note. Apple’s customer facing diagnostics are fairly rudimentary. Just presence checks for most hardware and quick checks for sensor data, short RAM tests and checking the SSD SMART data. So they wouldn’t always turn anything up.