Sonic Mania major marketing campaign was to showcase the game in a retro light, with the collector’s edition of original game including retro-themed items such as Sonic Statue on a Genesis console and a Sonic Mania cartridge. The release of Sonic Mania Plus was supposed to include a reversible cover that showcased retro artwork but it seems that Xbox One owners did not get the cover.
Source: N4G PC Xbox One Copies of Sonic Mania Plus on Amazon Shipped Without Reversible Cover