It's 110% an iOS 13.4 issue affecting iPhone 7. There is a video out there that showed this issue even in the beta (https://www.reddit.com/r/iOSBeta/comments/f0yrtp/bug_cant_watch_browser_video_on_134_beta_7_plus/).
Furthermore, it's broken on all browsers, not just safari. It doesn't work on chrome or firefox either. Doesn't work even if you request the desktop website.
I downgraded my phone to 13.3.1 and the video returns and works perfectly. Upgraded back to 13.4 on a clean install and the browser video breaks again. Not related to wifi, or poor connection, or anything else. I've tried restarting, erasing all settings, restoring the phone etc etc.
Absolutely infuriating bug, especially seeing as that it was present in the beta which was released over a month ago. Plenty of time to fix this nonsense issue. No reason for this to be happening. Bad oversight by the software development team. Guess this is how they force upgrades on consumers...