YouTube App loses video when I start playing certain YouTube videos.

The video may be in a format that your TV/Roku does not support. The player on the Roku and the one in the Web Browser are different. That is why it works in one but not the other. If your computer supports it, you can try Casting the video to the TV. Also, if it is a bug, you will need to raise the issue with YouTube Support. Roku won't be able to help you. Good luck!