A couple questions about what you are seeing - so the only thing corrupted is the payload? The packet as a whole is accepted by Losant, on the expected topic, but the payload is not what you expect? Is it possible that it isn’t corruption, but an encoding mismatch/display error? If you are sending binary data, the application log could display it very weirdly (the browser may be coercing the data to UTF8 when trying to display it).
There is no built in mechanism for CRC or checksums on MQTT that I know of (I think it just relies on the checksums stuff built into TCP), so Losant doesn’t have anything built in for that. It is certainly a thing you can add to your payloads, though, and then verify the checksum, hash, or CRC using a workflow.