In the scenario you described, the user should be taken to iTunes on the Desktop, or Apple Music on iOS or Android to start their trial. Is there a specific platform you are seeing this issue on? Which OS and Browser are you seeing this issue?
I got the issue on Windows 10 with Chrome browser.
On an iPhone with Chrome it seems working, but the affiliate links is not working anymore (the click is no more registered in the iTunes Affiliate Program, as it worked some days ago).