The source files have different solidity compiler ranges referenced.  This leads to potential security flaws between deployed contracts depending on the compiler version chosen for any particular file.  It also greatly increases the cost of maintenance as different compiler versions have different semantics and behavior.
This defect has numerous surfaces at https://github.com/code-423n4/2021-05-nftx/tree/main/nftx-protocol-v2/contracts/solidity
Recommend fixing a definite compiler range that is consistent between contracts and upgrade any affected contracts to conform to the specified compiler.
0xKiwi (NFTX) commented:
