Mosaic: Customizing an existing custom layout wipes out manual additions

Is there a way to ensure that manual changes to a mosaic layout are not clobbered by the
wysiwyg editor when making customizations to the layout?

Here's my experience...
I created a custom layout with Mosaic and then further customized it using the Mosaic Layouts Editor.

When I go back to the wysiwyg and and make changes and save as a new layout the manual additions do not persist.

I had the same problem when using Mosaic package...