USB On-The-Go

I have a power bank that is PD capable, but I cannot charge it from my MacBook even if the MacBook is plugged in to power. I get around it by using a USB-C/A dongle and corresponding USB-C/A cable. Presumably this "downgrades" the connection and since the MacBook doesn't support traditional USB charging it has to charge the power bank. 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?

Funny enough, if I plug the USB-C/A dongle on the end of the power bank and the cable into the MacBook, it also won't charge.

I also have a Philips One toothbrush with a USB-C charging input. Similarly, I can't charge it with a USB-C cable directly from my MacBook but have to use A in between (I unsuccessfully tried using either a thinner "lower speed" or a thicker "higher speed" USB-C cable). I'm assuming the toothbrush doesn't support PD, so then why can't it fall back to traditional charging with a C-to-C cable?

It's even simpler, I think: you're forcing it to avoid a PD negotiation, and fall back to the lowest common denominator, "500 mA on the PWR line supplied to the USB device". This goes, I believe, as far back as USB 1.1, as a slow-but-generally-safe power source for things that are barely more than "use USB PWR and GND as a dumb 5V source."

A C-to-C cable, OTOH, doesn't have this requirement, and if there's no PD negotiation, the MacBook is not required to provide power IIRC.