> Which source? The charger or the device? Because either could be at fault.

IMO it's harder for the source to be flexible with voltage, and especially current limit, so I'd put somewhat more blame on the sink here for not being flexible enough. Generally a sink should expect to be plugged into an 'unknown' source, so should at least make an effort to support multiple voltages.

Designing a multiport power source with a shared power limit is a bit of a tricky problem.