Strange error when upgrading contract (UUPS)

When I upgrade using upgradeProxy(proxyAddr, V2Factory), I am greeted with this error:

Error: New storage layout is incompatible

ERC721BurnableUpgradeable: Deleted `__gap`
  > Keep the variable even if unused

I am not even importing this contract (Burnable) and have not even modified any logic/ function/ variable in the contract. Just modified a string returned by a function to test out the upgradeability. I also checked and found that the variable __gap is still present in the ERC721BurnableUpgradeable contract (even though I am not importing it anywhere)

:computer: Environment


It seems like you update the storage, I am not sure, if possible, you can paste your original contract and the upgradable contract at here.