A 2nd suggestion:

Did you use wasabi (or similar) and then sent the btc directly from these mixed utxo to the exchange?

Because someone told me they analyze max 1 or 2 past movements from the utxo you send from. So if you simply move them around couple of times to other utxo you control yourself maybe they will not be classified as “mixed”

But I’m not sure about this, haven’t had any practical confirmation. Anyone tried this and can confirm or deny?