>Since it's designed to study the disincentives to work, by the end of the study it's likely to include many employed people.

On the other hand, people realize that UBI may go away any day.

What will be when canceling UBI has as much chance as canceling Minimum wage?

One would hope that the participants have been given a long term guarantee that this program will apply to them, perhaps 5 or 10 years. Without the guarantee, the results would be unreliable.