Someone pointed me to this thread from another channel. The solution presented above uses a join that includes all values selected, including the one selected in the question whose choices are being filtered. I believe that upon submission all values selected will always be cleared. It will appear to work during form entry because the filter is applied when determining choices to display and then not recomputed during further form entry. However, at form finalization time, everything will be recomputed and all values will be discarded.
I believe you'll need to do a join inside the repeat that excludes the current repeat instance's selection.
@ahblake were you able to get things to work as you wanted? If so, would you be up for sharing an example of what you did? If not and it's still something that would be helpful, I can take a look at how to approach it.