I have Opposed the Flag because you have no evidence. I've seen mixers that could use a Letter of Guarantee, but choose not to provide it. That means they don't provide the customer with the means to prove foul play, and that means they're partially to blame for having a scam accusation without evidence.
In this case however, MyCryptoMixer provides all users with a signed Letter of Guarantee. You could (and should) have saved the LoG before making a deposit. Therefore, the lack of evidence is entirely on you, and I'm not comfortable Flagging a user because of that.

I will however leave neutral feedback with this topic as reference.