@oelof Heyho, I also was curious about the charging current so i measured it using an USB Amperemeter (or Multimeter, you can get these rather cheap on amazon etc., looks like a USB Stick with a display). Anyway, the maximum charging current of my Traverse Alpha was 80mA, which is rather low. The problem with some power banks is, that they assume that the plugged in device is fully charged once the charging current drops below a certain threshold. In your case, this threshold might by above 80mA, so the charging mode of the power bank is not triggered since it thinks the device is already fully charged, due to the low charging current of the watch (as mentioned by andrasveres above). Portable solar panels might show the same behavior.
If your power bank has two or more inputs, you can try to confirm this by plugging in your smartphone (or something similar which accepts a larger charging current) to one plug and the watch to the other to prevent the power bank from stop charging.
Also a word on available charging currents: In case of the Traverse watch, it makes no difference if your charger can provide 2A or 1A. The load (watch) will take the current which it needs (80mA). A “stronger” charger wont speed up the charging of the watch. This is only valid for devices which drain more than 1A.
Hope that helps. Cheers!