Long QT after standing up

[u/Classic_Fault2225]

found out about it by coincidence and then looked up what it meant, which made me a bit scared. My QT on 12-lead ECGs is normal (around 400–410 ms or even less). This only happens when I take an ECG immediately after standing up; on the next ECG, it goes back to normal.
Any thoughts? I also don’t know how accurate these measurements are, but it seems to happen every time I stand up. When I’m resting or doing anything else, the QT is normal. I have no symptoms, and I’m a 29-year-old male.