They can sell whatever rights they want to whomever they want. They can most certainly lose the rights if they choose to sell them.
I think you're thinking about licenses. Selling a license is a different thing.
IP law's pretty clear on that. The main source of disputes is evidence that rights have/have not been sold.
Author never loses copyrights in most civilized nations. Period. These days it's normally at least until their death, and in many nations, long after they are dead.
Now the rights of sale (which is what most are referring to) ARE what you are referring to as rights. Those are not the same as copyright however. Depending on what is involved with those contracts, will determine exactly what rights go with that right of sale. For example, Tolkien did not sell the movie rights to the same folks who had the book rights. In addition, there were additional rights which allowed other authors to be attributed to if they wrote under those rights of sale.
However, Tolkien and his heirs still own the copyright, and as such, still own the LotR overall. Sure, they may not be able to do anything with them as the rights of sale are controlled by others, and to try to do something outside that contract would break contract and open them to lawsuits...but that does NOT invalidate their own copyrights of ownership.
Even AFTER the copyright or ownership has finally expired (and this lasts for different amounts of time depending on where, for example, copyright in Australia lasts less time than in the US), it can be odd to determine who or what is the official storyline.
I think one of the closer items to D&D one can look is Conan the Barbarian. REH's Conan was of a particular style and substance. However, the rights to that character were bought by another, and that other actually wrote their OWN books in regards to Conan. Later other authors wrote Conan stories (inclusive of Robert Jordan). So, the question then could come, which is actually Conan. Which is the official Conan.
Is ALL of it, including the stuff that directly contradicts the original creator's Conan? The movies seem more akin to the REH's Kull rather than his Conan in the character that is in them in some ways, and the storyline does not seem to correspond to REH's Conan at all. On the otherhand, it does correspond to some of the books written by others.
Who then, is Conan?
IS REH's Conan, the original and real Conan, the authoritative take on Conan. It was his creation and his character, thence, shouldn't his wishes and designs on what and who Conan should be, be the authoritave take.
Or should it be under someone who merely bought the rights after Howard's death...and then created their own stories considered the authoritative person to decide what and who Conan is, even if it flagrantly discards and in some cases despises the original author as some see the movies (starring the great Arnold of course) doing...be considered the official Conan?
Do you spit on the original author and say he was an idiot and you hate his Conan? Or do you respect his writings, as there would never have been a Conan to begin with otherwise, and say that whatever his take was is the authoritative take on Conan. The others are also other renditions, but the true and final one is Howards.
Or do you say, that was a Conan from a LONG time ago, and more modern writers and authors have come about. It was THEIR writings and publications which actually made Conan popular, and made him an icon. Hence since their version is the iconic form of Conan, (as Howards was basically lost in obscurity before the republished it with their own editings and then created their own version of Conan), Conan is truly their version and the one that should be considered the OFFICIAL Conan.
This is also the situation with D&D I think currently, at least with the fanbase that plays it in the majority now. Most of them know D&D from WotC's D&D, and not Gygax's...but Gygax and Arneson were the original creators.
In these cases, though you may think YOUR OPINION is the absolute and black and white, I don't think there is any clear cut black and white. I think that there will be those that will say...the official Conan is REH's...and all others are pale imitators that stole the name for profit...whilst others will say REH's Conan was lost...the REAL Conan that was made popular and that everyone knows is the one that is and should be the official one.
When you get into the same IP but with drastically different versions...there is no clear cut black and white in my opinion.