In an Email Template or Signature, when using the HTML Code View in the Editor to make any changes directly using the HTML, the changes are not being saved.
The Template or Signature is being closed while the HTML Code View is still displayed.
When adding or deleting anything in the HTML Code View, always close this to re-display the main Editor pane before closing the Template or Signature. This ensures that any changes made directly to the HTML are saved. Refer to Creating the Email Signature using HTML and Adding Merge Fields.