Not sure when this first started, but it’s been happening for at least a few months for me. In Safari, on an iPad Pro (10.5” - latest OS version) - if I pause a video in full screen, and the iPad goes to sleep before I resume the video’s position isn’t saved properly. Instead, it looks like the video gets marked as if it’s been played 100% (so even if I had a previously saved spot it’s starting at the beginning again).
Also seems to happen if I exit full screen, but leave the tab open when the iPad goes to sleep. If I exit the video and close the tab my state is generally saved where it should be. My guess is that some event being triggered when the iPad sleeps is being treated the same as a end of video event.