I don't get stand reminders but I get alerts when I close the rings... even though it's set not to alert me. I've never had that switched on so after the last update it's just decided to do that and not remind me to stand. Recently contacted Apple Service through chat for a different Activity app. issue and they checked the hardware on both my phone and watch and confirmed all was fine.
Yes, the wheelchair setting is set to "no."
Yes, I've restarted my phone after this.
Yes, I've unpaired and repaired my watch.
No, it's not set on 'do not disturb.'