Submitted by 0x29A
The EIP-2981: NFT Royalty Standard implementation is incomplete, missing the implementation of function supportsInterface(bytes4 interfaceID) external view returns (bool); from the EIP-165: Standard Interface Detection.
A marketplace that implemented royalties could check if the NFT has royalties, but if they dont, add the interface of ERC2981 on the _registerInterface, the marketplace cant know if this NFT has royalties.
Like in solmate ERC1155.sol add the ERC2981 interfaceId on the FERC1155 contract
aklatham (Fractional) confirmed 
HardlyDifficult (judge) commented:
