System Priority = 0 is a special value.
In the spec of 802.3ad, while there is no EXPLICIT section that states
that 0 is an unacceptable value, there are sections where it says that
zero means ineligibility or absent/not present or disable.
What is explicitly mentioned that the system ID which is composed of mac
and priority cannot be all zero. So the prio >0 in XR (and IOS based switches btw)
will allow for accommodation of that clause.
Many OS's do not allow for a manual configuration of zero, but some (eg extreme) send it by default.
In short, a system priority of zero should not be used in general with lacp.
regards
xander