Since Canvas released its new Rich Content Editor (RCE), we have run into issues with updating page navigation links using the Canvas RCE. The issues include breaking the page style or removing the icon.
To work around the limitations of the Canvas editor, we recommend using the Design Tools sidebar to update the navigation links for pages created with Design Tools. For the correct process of updating navigation links for pages created with Design Tools, see How can I update a front page navigation link without breaking the style?