As you're touching on, it doesn't make sense to make a blanket statement for all phones. Also, iPhone is not USB-C, so I guess it doesn't participate in this negotiation.
What about Lightning to USB-C cables? I assume the device is talking the USB protocol at that point? Or does that connection only allow charging in one direction?
I've charged my iPhone off my MBP using a lightning->usb-c cable and I didn't notice anything weird. I've also been able to charge my iPhone off my iPad using the same cable so maybe it is one way.
That particular cable only works in one direction. Lightning itself can power external devices, but the whole mechanism is completely different than how USB PD over USB-C works (it is designed for low power peripherals and thus can output unregulated 3-ish V in order to save on DC/DC conversion losses, similar capability of devices powered entirely from V_CONN is quite recent and hackish addition to USB-C).
Do you mean phones here or iPhones? I've been charging my LG phones (G6 & G8) from MacBook Pros for years and never noticed an unexpected switch.