ms office – Word document reverts to all default styles when pressing Cmd+z

I have a heavily formatted document with several custom styles. However, when I press Cmd+Z to Undo, the document reverts to all default styles, a drastic change that I then cannot undo anymore.

This appears to occur only after I have first (inadvertently) pressed a certain key combination that indents a paragraph and add enumeration to it.

Does someone know what is going on here?