This was handled very poorly by PU and U3. I probably would not have made that call if I was U1 ... but if I had, I would expect to be backed up by my partners, not overruled. And if a partner had (even if I was U3), I would have not even interjected myself into the conversation - and if pulled in by U1, I would have offered what I saw, and left U1 to make or reverse the call.
I can CERTAINLY see how U3 in foul ground could have missed the OBS, and a U3 out of position should not be complaining about an OBS he easily could have missed.
And you know what ... I might have walked off the field as U1 as well. So blacklist me then.