I am not with Laminar Research; just a flight simmer.
The arrangement you query was a special deal to encourage the purchase of XP10 with the sweetner of then getting XP11 for a low price overall when it was released. At that time XP11 was about to be released but the release date was uncertain.
Once XP11 was released the "buy one, get one free" deal ceased with the purchases like yourself taking up the offer with the freeby. Any new purchases were for XP11 and had to be purchased as a new product. There are many comments in this forum about the confusion.
If you think you have been "hard done by" then contact Laminar Research at firstname.lastname@example.org . When you do make sure you provide receipt numbers, date of purchase, method of purchase etc to confirm your purchase.