I'm running Safari 14.0.2 on Mac OS 11.1. Safari does lower background javascript execution, it allows it to save on battery life for background tabs.

I want this. I want it to save battery life.

So Twitter should make sure their javascript executes correctly when it is foregrounded and Safari does start executing it.

Either way, I honestly don't care too much why it is broken, Twitter could have sent a pre-rendered page down.