I just thought of another possibility: Someone trying to deanonymize the user could have deliberately sent them the 0.40156916 Bitcoin out of a coinjoin, hoping that the user would later send it to a KYC exchange, which they did.

This seems very unlikely though, since a much smaller amount could have been used. (dusting attack) Also if Wasabi Wallet 1.1.6 functioned correctly it should have marked this coin as non-private to the user.

Anyway this scenario seems very unlikely.

By the way, modern versions of Wasabi Wallet only show a maximum of 1 UTXO per address (even if the address actually holds multiple UTXOs), preventing you from spending multiple times from the same address.