@TeamYouTube watch history has been wildly inaccurate. Videos don’t register as being watched most of the time. If I watch halfway then click another video, then go back, it starts from the beginning instead of where I left off.
That's odd, could you file a feedback report with these steps: https://goo.gl/1ckkO5 ? We review all the feedback sent to us, but YouTube doesn't necessarily respond to every single one. We use the info to identify and fix issues, as well as track requests and make improvements.