Drupal Commerce Product Pricing Rules disapear
It sounds like for some reason the event is getting removed or changed to something other than "Calculating the price of a product." However, without seeing a screenshot of your Rules list, there's no way to tell. Look at the list and see what the event is getting set to.

If you use the product pricing rule add link from the product pricing interface, it should be altering the form to pre-select the proper event and hide the select list. Perhaps you have some other code on the site that undoes that selection or otherwise invalidates it.