I don't think that &ua_redirect= parameter is anything we support at our end, so we're probably just treating it as an unrecognized parameter and passing it thru as-is to the merchant's landing page -- i.e., when an affiliate hop link redirects a buyer to the merchant's landing page for that link, any extra parameters the affiliate has added to their link that we don't recognize would just get appended to the end of that landing page URL, where custom scripting in the merchant's page could recognize and do custom things with it.
This means &ua_redirect= may only work on a specific merchant's site, so you'd need to contact that merchant to figure out why it's not working for you, and you could not expect it to work on hop links for any other merchant's affiliate program. Ordinarily, merchants would simply enable product-specific hop links in our system if they want to offer their affiliates referral links that redirect to particular products' sales pages.