Recently OpenZeppelin changed the variable storage for all of its upgradeable smart contracts to use Diamond Storage. This is a major change.
Here is the merged pull request: Use namespaced storage for upgradeable contracts by frangio · Pull Request #4534 · OpenZeppelin/openzeppelin-contracts · GitHub
Here is the issue about it on Github: https://github.com/OpenZeppelin/openzeppelin-contracts/issues/2964
Here is an article that describes how Diamond Storage works: How Diamond Storage Works - DEV Community