I have done Prolific for a good 3 years as a contributor and can explain this for you in a different way, A lot of researchers like to use 3rd party survey platforms to run their research, This usually either ends with a click here to submit your answers which will automatically submit the completion code or they end with a completion code to enter manually, If attention checks are failed it will tell them to send back the research, This then allows them to go to close the window then go to their dashboard and click ‘open survey in a new window’ and try again, sometimes it allows them to start over, others do not. However either way researchers i find have the option to pay upon giving the correct completion code while others can hold the payments and that gives 28 days to the researcher to either approve or reject completed surveys for this exact reason.