You need to read the T/C, FAQ’s or ask the affiliate manager what the “number of return days” (Expiry date) is set for and if it credits the first or last affiliate. (ie… gets overwritten)
(For instance, the poster giant Allposters.com has a 10-day return cookie, however the most recent affiliate member earns the commission.)
If no Expiry date is set, the cookie gets deleted when the browser session is closed. (ie… a temporary cookie)