Do an online search for
apple watch manually enter standing
and you will see many options to do this. Ignore ones from more than a year or two ago as some of those don't work anymore but there are several ways to do this and some videos online to show how. Some of them involve manually adding a workout of a type that you can select during the time you are targeting, which can then credit the stand hour.
t sounds like this does matter to you but I am more relaxed about these credits and closing activity rings and the like for smart watches ... whether the watch credits you or not, you still did the activity and that's all that matters to your body and health, not that the watch captured it. Several times I have forgotten to start a workout and the watch speaks up several minutes in, "are you doing a workout" and presents a button to start it. So it gets started late but even so, one still did do the full workout. I try not to sweat the small stuff ... Don't let the watch add stress to your life, it should make you happier and more relaxed and lower your blood pressure, not raise it!
P.S. If it missed you standing once in a while, no big deal, but if it is repeatedly missing that, this could be something to troubleshoot further. I don't know how the watch can tell that one is standing, but my guess is that for a device strapped to one's wrist, it's not a perfect detection system and once in a while might miss something. I'm amazed the watch does as well as it actually does detecting these things.