This kind of feels like an issue on SmartThing’s side. It’s probably writing 100 to the switch, but according to the parameter list, the switch is expecting a value of 0-255 (0 = off, 1-254 = level to turn on to, 255 = return to prior state).
Does ST permit you to write a value > 100 to parameters #13 and #14?
100/255 ~= 39.6%, so that might explain where that number is coming from.