I just tried this out on both of my Mother’s and got the same response.
I have a 16 note sequence running and then I make the first step into a rest. Trying to make it a note again doesn’t work until I make step 2 a rest, remove the rest from step one, take it out of record then back into record and remove the rest from step 2.
Just tried this out and this works with mine. While playing the pattern in step mode, I can change the first step into a rest and back by simply turning it off and on as described in the Step Mode Tutorial, and I can also change it if I enter the edit mode for the first step and do it with the “Hold/Rest” button. It doesn’t matter if the 2nd step is on or off, and I can play around with the other steps as well.