The Froala editor is picky about what types of HTML elements are supported in it. For non-supported elements, they will be stripped out and/or converted to a element. Unfortunately, there is no standardized list of what is or isn't supported, but elements are one example that will get stripped out/modified.
If you are running into issues with unsupported elements, the recommended action is to use the Custom HTML Widget, which has far fewer limitations (but still does have some).