I/O pin driving PCB? Problem, Bug, Feature?
Yes, this is a pretty standard situation when a low-power IC gets powerd via a GPIO pin. All (I repeat, ALL) CMOS circuits carry ESD prodection diodes , which clamp I/O pins to ground and IC's power rail, see this picture from EE News Automotive, as example:



When the main rail is not powered and floating with high impedance, a voltage from connected I/O pin will have the upper clamp diode in open state, and I/O voltage will leak into power rail up to input voltage minus 0.7V of the diode, making the IC sometimes fully operational.

To prevent this kind of problems, a passive "bleeding resistor" (rail to ground) is usually sufficient, or, if extra power is of concern, it should be controlled with active switch (FET).