FWIW, I have a multi-head USB cable (handy for travelling or touring) that has a convertible USB-A/USB-C tip at one end, and four tips at the other end: USB-C, USB Micro B, and two Lightning tips. Only the USB-C tip carries data, and is marked with the USB logo; the others only carry charge, and are marked with a lightning bolt.

On the original subject, I have a relatively nice Lezyne headlight that does not operate while charging. Lezyne does sell a special external battery to extend its runtime, but that plugs in through a barrel connector rather than the USB port used to charge the light.