For the warning message, you can check whether same field is actually used in the criteria and target of the field rule as below.

For the issue the field rules disappear, it may because the field rules was not saved successfully, otherwise you can use following query to check whether there are field rules saved in the database(replace the {form_id} with the number you get from the URL when you open the field rules page of the form):

 select * from [cf_rules] where form_id={form_id}