Rich text editor is an old widget with custom implementations and based on an outdated library. This makes it hard for Mendix to maintain this widget and the reason why it isn't actively supported. So I would not encourage you to use that widget.
Does this widget bring you functionality which the CKEditor currently lacks? The CKEDitor also has a viewer widget and is covered with a better support category in the App Store.
Update1 Strange that the CKEditor also causes this behavior. Is it difficult to reproduce this? You can also try the Format String widget for the job. It has a property 'Render as HTML'