It has to be a defect in your watch. Some people complain that it tells them to stand when they have been standing for an hour but they never moved or walked more than a few steps.This really is ridiculous. I have it telling me to stand and move while I'm up walking around (and have been for minutes to hours) or it will tell me to stand and I do, walk around for 5 minutes and STILL can't get it to detect my motion! So incredibly frustrating.
My watch tells me to stand if I haven't in the first 50 minutes of an hour. If I walk around for 20 seconds (maybe less) it tells me Good Job and marks the hour off. I've even tested it by moving my arms for 20 seconds without getting up and it works.
I've stood at a bar for an hour and it asks me to stand and they would be correct. Maybe they should say "stand or move dummy" ;-)
Yours sounds like a flaw unless you walk without moving your arms or stand still. But, after reading what you wrote, I would say your watch is defective.
I'm at the office early today and at 6:50am it told me to stand. I got up and walked to the break room and before I got there, it said good job you did it. About 15 seconds actually.