I've yet to encounter these also. I've assumed it's just been luck with the A/B testing.. But perhaps it could also be that I've never allowed doubleclick.net scripts to run in NoScript, or that my ublock-origin setup has taken care of itself, or that because I've actually participated in youtube's other non-premium monetization schemes (channel memberships) from which they take a 30% cut and they want to upset somewhat paying users last.