Your performance may be defaulting back to layer because of something in your sequence.

Do your original test (i.e., with the performance you have changed to single), and after the sequence plays, check to see if your performance is still set to single.

If it has defaulted back to layer, something in your sequence (e.g., a performance program change message) is telling the synth to go back to that original performance. Unless it has been saved to a user location, it will always default to its original state.

Also: try saving the performance as a single, then playing the sequence back so it can't default back to a layer.

There are other possibilities, but check these first.

Good luck