ModX: Keep losing formatting when I return to edit a page

Question

I am running Evo 1.0.4 and often add HTML to my pages content by switching off TinyMCE.

I have an issue where if I return to edit a page at a later date, the HTML markup will be stripped out. The content will still be present but just wrapped in a single <p> tag.

This occurs with both mark up I add manually and mark up added by TinyMCE. Oddly enough, comments I put in still remain.

Would anyone know why this is occurring and how to fix it?