Studies typically have a set duration. If you offer people a basic income for the duration of the study, they'll need to plan differently than if they have some reasonable chance of having a basic income indefinitely.

This is certainly true. However, for many cases there may be little difference. A year or two of basic income is sufficient to start a business, learn skills to enable a career switch (exceptions, of course), care for a newborn, deal with short-term illnesses, etc.