So I charged it to 100% using a USB cable, then let it drop below 80 again, put it on the charging dock, and again it goes to 90% and stays there, even though the 80% option is activated. I think I will just wait this one out until it is fixed. There has to be some buggy interaction with the 2 different charging limits on the Pixel Tablet (80% limit, and the standard 90% limit when charging through the dock).