I deleted my original post because I suddenly got it working and thought it was a PEBKAC problem, but then I tested further and discovered more.
So here it goes, again:
When I drag a new ui_switch node onto my flow canvas and change the On/Off payload values from boolean true/false to number with On payload value 1 and Off value 0, these are not accepted when clicking Done.
Re-opening the node shows the values again as true and false, which is not accepted as a numerical input.
Any other value in either field works but initially I didn’t test with other values.
After using an arbitrary numerical value, clicking Done and re-opening the node, the 1 and 0 values are allowed as On and Off payloads 