After restarting system or DM the order of my panel's custom objects is chaotic. However I can restore my original order by killing the mate-panel instance.
`pkill mate-panel`
When it restarts the order is restored to where I had put them before the restart.
It's been like this since at least 16.04 -- when I started using UM. Not a big deal obviously, but I'd like to know why this is happening.