Sometimes it comes down to luck with the choice of usb/uart adaptor. Some may have resistors in the TX/RX paths and sometimes the TX/RX leds are powered directly from the TX/RX pins affecting their use as a updi programmer. In the latter case, removing the leds (or their series resistors) helps.
You can follow the story here which includes successes and failures with a selection of such adaptors. Yet another UPDI programmer Mk II
EDIT
The FTDI FT232RNL chip used on the USB/UART chip in the OP is unlikely to drive the leds directly from the TX and RX pins because other pins on the chip can be configured to drive the leds. Dataheet (PDF): https://ftdichip.com/wp-content/uploads/2022/07/DS_FT232RN.pdf.
Where did the idea to use a single 5.1k resistor originate ?