This issue is not a "flaw". It's a root problem. We want to know the effect of permanent UBI, not of a temporary one, and we know (strongly suspect) the effect will be different.
Maybe the simple solution is that the researchers establish a dedicated
million dollar bank account for a participant and automatically withdraw $1000 for the participant every month.
Maybe the simple solution is that the researchers establish a dedicated million dollar bank account for a participant and automatically withdraw $1000 for the participant every month.