Currently, the editor strips all classes/styling if content was updated from the WYSIWYG Editor after styling has been applied from source.
Please see an example here of this happening:
https://www.dropbox.com/s/9go4semst5y53lm/builder-editor-bug.mov?dl=0
As you can see, the current behavior results in dataloss and isnt ideal.
This has been a pretty significant pain point for us as well. It has caused us to push some bad pages to production and generally slows down our development.