Any type of liquids when spilled, boiled over or splashed on the door or door glass can find their way down in between the door glass. The cleaning instructions for the oven door are shown in the first image below. This is an excerpt from your owner's manual. It indicates that you should not allow water or glass cleaner to enter the oven door vents. This principal also applies to other liquid spills such as grease. Grease or other spills should not be allowed to enter the oven vents. The spillage may not be apparent until the oven is put into a self cleaning cycle where the temperature will get extremely hot and then the stain becomes visible. The self cleaning feature is not going to be able to burn off any spillage inside the oven door.
Here is how the self clean feature works in this and all other ovens:
The control locks the oven door.
The oven is heated to extremely high temperatures above 650 degrees.
Grease and food soil that is on the metal bottom, top and sides of the oven cavity will be burned to a white ash that can then be wiped out of the oven after the self clean cycle has ended and the oven is cool.
Limited areas of the oven door may be cleaned. The metal parts of the door around the glass will normally be cleaned during the self cleaning cycle. The inside glass will not be cleaned by this process. The self clean feature is only intended to clean the metal inside cavity of the oven.
I hope this explanation gives you a better understanding of the self cleaning process in this and all other self cleaning ovens.
Most ovens will have vents in the oven door that are vulnerable to spills that will soil the inside door glass. The door must be vented in order for heated air to escape when it expands. This is not an issue unique to Kenmore ovens.
I understand your disappointment with this Kenmore self cleaning oven. Although I cannot totally resolve your problem, I can attempt to explain why the interior glass gets stained and will not be cleaned by the self cleaning cycle in this or any model of oven. For help with customer satisfaction issues, I recommend that you call the Sears National Customer Relations Office at 1-800-549-4505. They will be happy to help you.
If you want to attempt to pull the oven door off of the range and take it apart, I provided some information regarding this procedure in the second image below. If you take the oven door off of the range, you can disassemble it using the parts image as a guide. I recommend calling a service technician to pull the oven door off and clean it the first time. If it ever needs to be cleaned again, the information below should help you complete the procedure.
If you need more technical help with this problem, resubmit your question with additional details and the full model number of your Kenmore range.