Since I can remember the insertion of the hexadecimal codes in the Discourse palette has always been exactly as you described it. You must manually enter only the hexadecimal number and omit #
Many color pickers give you the option of copying the hexadecimal code by previously deciding the code format
Sure I get this, it’s just that this behavior is a bit odd. I had to check the value I was coping pasting a couple times before noticing that the last number of my string was missing in the input. I didn’t understand why the value was invalid at first and I guess this specific copy paste behavior could be improved with no drawback.