(not sure it's really due to either plugin, vs. jenkins native behavior)
I was using a custom workspace, but no longer need that.
So, when I un-selected "Prepare an environment for the run"
The multijob continued to create/use the "custom workspace"
I had to re-enable "Prepare an environment for the run"
un-select "use custom workspace" in the Advanced options
then disable the "prepare an environment for the run"
and now that custom workspace doesn't get created every time jobs run now.