i'm not sure if this is something on my end or google's, but i've noticed that in the past few weeks whenever I open a youtube video in a new tab (staying in the current tab), it won't play until i switch to the new one.
Firefox definitely does this, though it doesn't stop autoplay when opening youtube on a new tab (I'm okay with autoplay on the same tab, ie directly navigating videos or autoplay function in youtube itself.