FTDI (FT232RL) powers itself through RX/TX lines
Your FTDI chip is being powered through the input protection diode on the input line.

Although FTDI does not appear to have shown it in their data sheet the inputs of IC chips are very often protected from ESD by having a diode clamping structure that clamps input pin voltages to be between GND and VDD. When the supply pin is not connected to anything a high level signal on the input pin can source current through the upper clamp diode and "power up the chip".