I'm betting that the device admin setting is protecting v21.0 being overwritten by v21.3, but we'll see what they tell you.
My Note 20U is on version 20.8.
Edit: Well that's not the response I expected. They obviously didn't understand your question.
Edit Again: I never went past the startup screen on 21.3 when I tried to update 21.0, so I don't know what it would have done from there. I have the subscription for 3 devices but only using 2, so it might have taken the 3rd device slot.
Very cumbersome...