[ThrustVectoring]

This should result in differences in tax treatment between exchange-held and wallet-held balances.

If you hold it in a wallet, you have one asset that is dividable into two parts, each transactable on different blockchains. If you hold it on an exchange, you have the right to receive an asset on one blockchain, and after the fork the exchange credits you the right to receive an asset on another blockchain.

These are different events. Arguably, all exchange balances in new blockchains are basically "airdrops" of new economic rights. It's not directly held assets, but rather an abstraction over it, and this abstraction is different. If the exchange wants to, it can refuse to give you airdropped or hardforked assets on "your" crypto.