Missing purchased books from iBooks library

Help! I’m missing over 20 books (well over 100 dollars in purchases) in my iPad’s iBook library that I bought from 2012 on. There may be more, but these are my favorites and stand out since they are all from the same author. I talked to support, but they were unable to help, despite trying a number of things. I believe the books disappeared from my library when I had to move to a new iPad and restore from a backup. The books definitely show in my purchase history, and even more, the books are still sold on iTunes, albeit under a different publisher. The books still exist on my iPhone, but I can’t transfer them to my iPad. Support even tried to re-grant rights for the purchases in order to help with the airdrop or restore from apple’s side. Does anyone have any suggestions for either fixing the issue or who at apple I can contact about this? Thanks