> Does USB-C not have a way to indicate that a potential power source is a battery so that the MacBook can charge it if it's plugged in to power, and reverse roles otherwise? Is this a fault how the power bank or macOS implements power negotiation, or is this scenario simply unaddressed in USB-C?
USB-C does support this. It's known as a DRP (Dual Role Port). USB PD can be used to signal switching between downstream and upstream facing ports depending on state of charge of the battery for instance. The problem is that many devices do not support this, and I strongly suspect your power bank is the issue here. iPhones and Apple stuff in general supports DRP renegotiation quite well. They tend to be USB-C compliant as much as possible, which can lead to issues with interfacing with 'USB-C' devices that are not actually properly compliant.