Why did Disney lose rights to Winnie the Pooh?

Could you elaborate on the reasons behind Disney's loss of rights to the beloved character Winnie the Pooh? Was it a contractual dispute? Did the original creators reclaim their rights? Or was it due to some other unforeseen circumstances? Understanding the specifics behind this situation would provide valuable insight into the intricacies of intellectual property law and the complexities of managing licensing agreements in the entertainment industry.