I think it's a safari issue .I would do everything indicated in the page I'm linking to https://beta.apple.com/sp/betaprogram/unenroll, including leaving the program. Then I'd re-enroll again as if it was the first time.
If you've already done that or it doesn't work, you might want to restore your phone.
When I use google chrome on iPad , it works .However, I can't use chrome for enroll my iPad