Missing bitcoin... over 6 confirmations! Ok, so I sent a vendor 0.0046531 bitcoins using Muun wallet. The address I sent it to is 3LTJzNzbXDBDdzHPxDSQk6sucLDKPW3zmN. The blockchain explorers I have used to track the money says they were received and “spent”, leaving the wallet with a balance of zero. Therefore I still owe the vendor money, but what happened? Is it fraud? I’ve used this vendor for years and something like this has never happened before.
Missing bitcoin

Nothing seems to have gone missing.

Therefore I still owe the vendor money

If you sent the money to the correct address, you do not owe the vendor any money.
About 25 minutes later the money was spent in a transaction with about 55 inputs and 15 outputs, maybe the recipient has a custodial wallet whose custodian reorganised some of their holdings or maybe the recipient ran the money through a mixer for privacy.
However, it doesn't matter what they did with their money after they were paid, the blockchain shows they were paid.

leaving the wallet with a balance of zero

You have no way of telling the balance of someone else's wallet. A wallet can hold many addresses. One or more of the output addresses in the second transaction could well be in the vendor's wallet.
If you are referring to your own wallet I don't see how your remaining balance is zero since there seems to be change going to it from the first transaction.