SHEIN Removes Luigi Mangione Look-Alike Model Ad, Conducting Review

7:10 AM PT -- SHEIN tells TMZ Wednesday morning ... "The image in question was provided by a third-party vendor and was removed immediately upon discovery. We have stringent standards for all listings on our platform. We are conducting a thorough investigation, strengthening our monitoring processes, and will take appropriate action against the vendor in line with our policies."

SHEIN is speaking out after one of their ads appeared to feature suspected  UnitedHealthcare CEO shooter Luigi Mangione ... and it seems they're as confused as we are by the advert.

A SHEIN spokesperson tells TMZ the listing was pulled down as soon as they were alerted to our story, calling out the similarities between the ad model and Mangione.

As they told us ... they're unclear how this situation happened in the first place -- but are working to change protocol to make sure it never happens again.

Remember, we broke the story late yesterday, when Mangione's apparent mug surfaced on an advertisement for a men's shirt for SHEIN. Mangione's likeness didn't hurt the shirt sales, however ... the floral button-down was nearly sold out at a $11.69 price point.

SHEIN has yet to clarify whether the model in question just REALLY looks like Mangione, if LM may've had a past posing for the company, or if this was just an A.I. blunder. But they assure us that they're looking into it now.

For those wondering, Mangione is still behind bars at MDC Brooklyn, where he awaits trial for allegedly murdering health care executive Brian Thompson in December. So, we DOUBT he booked some modeling work on the side.