Are you using the link cable (prog box to esc) that come with the 1-2s esc? I had this problem also, when I used the cable that come with the 2-3s it wouldn't work. As soon as I used the cable that come with the 1-2s it worked fine.
If you look around the sensor cable end of link cable of the 1-2s you will notice they is some electrical bits sticking out under the heat shrink. This is the only difference I can see ,it seems to work with the correct cable anyhow.
HTH
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