I have now tested this issue in both the most recent version of Chrome and the most recent version of Safari. When adding content to a newsletter, the content is not displayed in the preview and not retained if the newsletter is saved. Further, the "Save" button at the top of the panel is 'greyed' out and the only way to attempt to save added content is through the prompt when sending a sample.
I have not yet had the chance to test this issue under older versions of WordPress or in Firefox (or under Windows). This may be an issue that's new to WordPress 4.2.2.
If needed/helpful, I shall grant access through the support option in WPMU DEV Dashboard.