I am on an iPhone 8. I view the site in Full Site mode. If I view in mobile mode, i don't get the red background, and I don't see embedded tweets.
From what I can find, ad or content blockers may prevent them. Also, in the Settings/Safari under Privacy and Security, the only thing I have selected is Check for Apple Pay. I don't know a lot about those settings, but I did read that having cookies blocked could prevent the embedded tweets from showing.
If you make any changes, you may want to clear your History and Website data. Good luck, wish I had a more specific solution.
“To me there is no dishonor in being wrong and learning. There is dishonor in willful ignorance and there is dishonor in disrespect.” James Hatch, former Navy Seal and current Yale student.