According to the documentation, the value of priority and risk should be 0 through 10 or "N" for None.
It looks like the value of N must be in quotes. That's fine. "N" and "n" are appropriately processed the same. This is now documented.
However "n" looks like it decreases the value by 2 rather than setting to None.
When using -p +x, it correctly does not allow the value to go below 0.
However, if an attempt is made to set the value below zero, it looks like "sometimes" the requested value is actually set, even though the UI doesn't reflect it.
I can't yet reproduce this on demand, but as I was testing. I did something like this:
-p 5 << establish a baseline
-p -6 << -1 invalid, correctly ignores request
-p +2 << usually returns 7, at some point returned 0 or 1
HTH
T