The Miyamoto Seal is a concept brought up shortly after the release of Sonic Gems Collection.
Shigeru Miyamoto, creator of game series such as Super Mario and the Legend of Zelda, isn't just a game designer, but also an oracle-priest who practices Shinto-Buddhist magic. The idea of compiling older games onto a single disk and releasing them to prevent piracy and emulation at the time was common practice, since virtual download shops weren't available yet. Going along with this practice, Sega came up with the idea for Sonic Gems Collection, a collection of more obscure games that would serve as a companion to the more mainstream-oriented Sonic Mega Collection. One such game was Sonic R.
Miyamoto had knowledge of the curse that was put on Sonic R, and upon hearing about the release of this game, he requested to see the first copy of the game, before it was copied onto other disks and mass-produced. He blessed it with a seal that would trap the Tails Doll within the game, hoping he would never be able to escape. This renders the curse impossible to perform on this version of the game.
It is unknown if the seal is present on the Playstation 2 edition of the game.
Breaking the Seal
Unfortunately, it is possible to break the Miyamoto Seal. While originally the typical way to perform the curse was simply to finish the game 100%, now by tagging Super Sonic on Tag Four Characters mode on Sonic R after completing the game 100%, the Tails Doll can gather enough power to break the seal and enter into the human world.