If I change the disk layout for a Category, and the node has role slurmclient, munge fails to start because /var/log/munge doesn't exist, Change the disk layout back to slave-xfs template, and munge starts. Any ideas?

Your change of the category may involve a change of the software image as well. When you set up Slurm, changes were made to the software image that the nodes belonged to. If you are switching to a software image for which Slurm has never been set up, then that would explain why the /var/log/munge directory does not exist. 

Note that there may be other reasons as well. One thing I would recommend checking is whether /var/log/munge exists in the software image that you are using. If it does not exist, then that's why. 

I find it strange though that changing the disklayout somehow resolves this, because if /var/log/munge is not present in the software image, then using a different disklayout is not going to resolve that.