Nodes that don’t know about Taproot need to be able to validate the blocks with Taproot transactions. Marking them as valid spends is the only option for not forking the network.

Yet, if the transactions are valid, the old nodes have no idea where those coins got spent. Are they seen as lost coins?

What happens when a Taproot transaction is followed by a old style transaction such that UTXOs are back to being available for spending as an old-style transactions?