I had the problem a few days ago on a few videos when using Chrome. I have just tested it today and everything is working fine on Chrome and Firefox.
But to be honest, it's hardly a serious problem. How often do you watch the same video several times in a row anyway? There is a very simple workaround by just refreshing the page anyway.
People on here can compare experiences to see if other people are having problems, but no-one except YouTube can officially confirm if there is an issue or not and they are the only ones who can solve it if that is the case.
Good luck