Submitted by cmichel, also found by 0xRajeev
The RCNftHubL2.safeTransferFrom function does not correctly implement the ERC721 spec:
This check is not implemented, it just drops the _data argument.
Contracts that dont know how to handle ERC721 tokens (are not an IERC721Receiver) can accept them but they should not when using safeTransferFrom according to spec.
Recommend Implementing the IERC721Receiver check in safeTransferFrom.
Splidge (Reality Cards) confirmed and resolved:
