I'm not having the exact same issue as the previous posters, but I'll add a recent issue I've found with the loop function that wasn't there before. I've noticed the shortcut of typing "A" and "B" to set the start and end points of the loop is broken is Safari 14.0.3 on Big Sur.

If you type A and B, it creates start and end points but not where the cursor is currently. Dragging and dropping the endpoints manually seems to work, though.

Being able to quickly set the A and B points with shortcut keys was a gamechanger when I read about them on the forum, but upgrading to Big Sur seems to have broke the feature.