@YouTube I have had Premium for about two months. I love it. However, today I saw my first ad on a video even though I was signed in to Premium. The ad came on few seconds after I started watching the video. Someone else here on Twitter also saw an ad today on Premium. Thanks.
Just to clarify, have you tried signing in on a different device to check if the same video/s shows ads? To narrow down the reason on why you're seeing ads, try the common troubleshooting steps here: https://yt.be/help/ADht Let us know if that fixes it.