Is powering the Nuvoton N76E003 with just the UART0 pins legitimate?
You're likely backfeeding the power supply through the ESD-protection diodes integated on the chip. They dissipate static charge that may be applied to a pin, sending it safely to the power rails (which are expected to have bulk capacitance, a power supply, loads, etc which can absorb that charge), protecting the very sensitive thin gate oxides of the chip's transistors from catastrophic damage.



simulate this circuit &ndash; Schematic created using CircuitLab
This happens to work, including in interesting projects such as this one, but it's almost certainly not an intended mode of operation designer unless specified explicitly by the datasheet. The diodes have a limited current handling capacity, and because your supply might be coming through a UART pullup resistor, it's possible to experience both diode voltage drop and resistive voltage drop related to the load current.