Yes I have weird issues with YouTube as well.
- A lot of time the Chromecast is detected on the Chromecast app on my phone, but Youtube doesn't have the 'cast' button available. It's as if the Youtube app on my phone doesn't detect that there's a Chromecast connected to the wifi network.
- Sometimes when the cast button on Youtube is available and I cast something to the TV, it will randomly disconnect Youtube from the Chromecast and the video will continue to play on my TV and then also play on my phone. When this happens, the cast button vanishes on the Youtube app. Again it's as if Youtube can't detect or simply forgets that a Chromecast device is on the network.
I've tried re-connecting the Chromecast, tried signing out of Yoututbe on my phone, closing the Youtube app, re-opening it, signing in, I've tried restarting my router etc but sometimes it just doesn't want to work. Sometimes it does work, and there are no issues, but then on another day one of these issues happen.
I'd say it doesn't work more often than it does work.