There are a couple issues going on here. I am guessing in Dev the solution is unmanaged meaning the user doesn't need any platform permissions to run the app/flow. In UAT I am guessing the solution is managed and the power platform wraps a layer of security to the solution requiring users to have  a security role in the power platform environment in order to run the flow. While modifying the Basic User role works, it isn't a best practice to modify a built in role. Try assigning the users the Flow-RP Role security role. This will give them the required permissions to run instant flows called within apps.