Short answer: Presumably the problem is that the watch is charging wirelessly with a proprietary magnetic charging pad, and this charging pad is either not getting enough power or not getting the detection it needs when connected to this 30W 3-Port GaN II Fast Wall Charger; the charger itself charges other devices normally, the watch charges normally with other power adapters, so the cause is an incompatibility between the charging pad/USB cable and this power adapter or a protection/detection function of the power adapter or the watch.

Main reasons and checks (short, prioritised)
- Charging method of the watch: The Galaxy Watch 8 charges *wirelessly* via a magnetic charging pad that is connected to a USB power adapter via USB cable; the watch expects a stable 5 V supply from the charging pad and correct charging detection from the charging pad/power adapter.
- Power supply ports and detection: The Ugreen 30 W power supply has 3 ports (1× USB-A, 2× USB-C) and uses GaN; some multi-devices try to distribute power across ports or negotiate differently via Power Delivery/protocol, so that no constant 5 V detection is supplied at certain ports, which the watch charging pad requires (especially if the charging pad does not use PD handshaking). Product specs show 3 ports, 30W total power, GaN technology, but no specific assurance for magnetic watch charging pads or low charging current detection for wearables. (Product data: 3 ports, 1× USB-A, 2× USB-C, 30 W, GaN Technology).
- Check cable / charging pad: Some original Watch charging pads only work reliably with certain USB output types or require the original cable; test the Watch charging pad on the problem power supply with a different USB cable/port of the same power supply (e.g. USB-A instead of USB-C or vice versa).
- Note the port type: If you connect the magnetic charging pad to the Ugreen power supply via USB-C, the power supply may try to negotiate PD instead of supplying stable 5 V; however, many watch charging pads simply require 5 V. Try the other port (USB-A) or a pure 5 V charging cable.
- Firmware / protection functions: Some GaN multi-chargers switch off very low impedance when the load is very low or unusual (like a smartwatch on detection handshake), so the charging pad will not initialise. The fact that other devices (power bank, mobile, tablet) work and the watch charges with other power supplies speaks for this behaviour.
- Simple tests you can do now:
- Use the original Samsung charging cable/pad on this power adapter on both ports (USB-A and USB-C) and compare.
- Test another USB cable between the power supply and charging pad (especially one that only supports 5 V, no PD handshaking).
- Check if the power supply needs another connected load (sometimes a second device needs to be connected to keep one port active).
- Observe the LEDs/indicator on the charging pad: Does the pad show power when connected to the Ugreen? Is the watch charging when the pad is connected to a normal 5 V mobile phone charger?
- If still not charging: Possible solution is to use a different power supply/port that provides constant 5V, or use a short USB-A to USB-C cable/adapter that bypasses PD negotiation.

Uncertainty note: I am not completely sure what exact detection or shutdown logic is implemented in this specific Ugreen model because the in-house product data only states colour, ports, GaN technology and 30W total power; therefore, I cannot state the exact electronics cause with complete certainty. However, the above tests are standard, tried and tested troubleshooting steps for wearables charged with magnetic charging pads.

If you want, I can send you a short step-by-step guide to testing (which cables/ports in turn) or formulate a text for customer service.