The PuttyV2.fillOrder will mint a long position NFT and short position NFT to the order maker and taker. When minting a NFT, the function does not check if a receiving contract implements onERC721Received().
The intention behind this function is to check if the address receiving the NFT, if it is a contract, implements onERC721Received(). Thus, there is no check whether the receiving address supports ERC-721 tokens and position could be not transferrable in some cases.
Following shows that _mint is used instead of _safeMint.
https://github.com/code-423n4/2022-06-putty/blob/3b6b844bc39e897bd0bbb69897f2deff12dc3893/contracts/src/PuttyV2.sol#L303
Consider using _safeMint instead of _mint.
