I've made what you said, but it does not (always) work.

The reason for is simple:
If you switch Auto Return=on AND PayPal Account Optional=on , PayPal does NOT return if the paying customer is a new one for PayPal - this means he or she is or has not registered himself or herself within PayPal.

It is obvious that in this case PayPal wants to show such customers a PayPal window telling them about how good it is to become a PayPal member instead of returning to the calling shop without using this advertisment.
You can find a description of this behaviour in the help text attached to the Auto Return Radio Button:
"Note: If you have turned on Auto Return and have chosen to turn on PayPal Account Optional for new users, a new user will not be automatically directed back to your website, but will be given the option to return."

Regards, tuttoBENE

Attached file contains the same text.