I answered my own question. It seems that by default when I add a new userform to the project it's ShowModal property is set to True.

When I set this property to False for the secondary UserForm, I get the desired behavior.

I had read other threads about issues with using Modal UserForms, and understood there to be issues at times. Perhaps this is one of them.

I hope this can help anyone else encountering a similar issue.

Regards,

Shred