Hi Magnus,
Unfortunately you found a bug. Here is a patch that will prevent users from making persistent job priority changes. We should probably return an error for this condition, but I would like to defer that change to the next major release, v15.08.
https://github.com/SchedMD/slurm/commit/4454316ef527b8700743d94c958811a39609e7d5.patch -- Morris "Moe" Jette CTO, SchedMD LLC Commercial Slurm Development and Support
