Acceptable workaround for me was to enable "Show hide buttons"Show hide buttons in the Panel properties (just below "autohide"autohide option, package mate-desktop 1.22.2-1~18.04.york0mate-desktop 1.22.2-1~18.04.york0).
This adds 2 small buttons on each side of the panel and when you click on either the panel gets hidden.
I rarely need to see that small part of the screen so this workaround is good enough for me.