I was having some problems with playback on youtube with "buffering", random skips, the video reloading, etc. It turns out that those pauses and skips were for ads that uBlock stopped. Channels with more ad placements(new videos from large channels, large companies) would stop more often. Looking at the logs for Ublock showed me that yt does track how much of the video you have watched regardless of where you started. Say I load a video and skip to the middle. It will do a callout for time watched.

I am not sure if I'm right but anyone else could correct or expand on this as I am no expert in how youtube does anything these days.