Editing callout boxes in an imported vsd file?

I'm running OmniGraffle Pro 5.2 on OS X -- and have imported a vsd file that
includes several comment or callout boxes.

The boxes are not rendered correctly in OG, and the application behaves
badly - it won't delete selected callout boxes, it won't perform a 'select
all' correctly, etc.  I was about to give up and re-do the diagrams when I
decided to try one more thing.  I saved the file as an .omnigraffle file and
then re-open it.   All's well in the world.

I don't know if this is a well known technique for import problems.  I
checked the FAQ's and didn't see it suggested there.  Might be a good thing
to add to the import Visio section.

Pauli