Unexpectedly drawing power over serial tx/rx? What to do?
simulate this circuit &ndash; Schematic created using CircuitLab

It is common for pins on a chip to be protected with diodes. These diodes clamp the pin to the rails. This way if a voltage transient occurs such as a static discharge the pin is undamaged. The TX and RX lines are pulled up with resistors somewhere. This may be inside the chip or externally. UART has to have pull-ups. What is most likely happening is that D1 is forward biasing and powering up the rail that it is attached to. You can increase the value of the pull up on the line to alleviate this effect.