The problem is that:

• the standalone WYSIWYG editor reloads (recreates) the iframe used for the editing area
• when you switch from Source to WYSIWYG without changes the editor simply sets the content of the (recreated) editing area to the previous HTML which has the MathJax formulas already rendered
• MathJax sets a property on the formula wrapper DOM element to know that the formula was rendered. DOM properties, unlike DOM attributes, are not serialized to HTML so they are lost when the editor resets the editing area to the prevous HTML
• As a consequence MathJax tries to re-render the already rendered formulas, but the content of the formula wrapper DOM element doesn't have the formula source code anymore, so nothing is rendered.