postfix set-permissions is a feature of postfix.
So, it this :
1) a bug in postfix -> should be reported and fixed upstream),
or 2) bug in Ubuntu package ? -> Ubuntu package should be fixed
How does a plain postfix installation behave ? Can this happen in a plain, unpackaged postfix install ?
If yes, it is a bug in postfix. -> 1
If no it is a bug in Ubuntu package. -> 2
Based on googling the error message, I suspect it is 1, can anyone confirm ?