(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.