When I transfer an ERC721 token from the DappChain to Rinkeby using transferGateway.withdrawERC721Async and then mainnetGateway.withdrawERC721, it takes a long time before I can transfer another token because I get the pending withdrawal error. Getting the receipt tells me that the token I already successfully transferred is the pending withdrawal.
When I call withdrawERC721Async, two identical transfer events are emitted, I don't know if that could be related.