Stock its a diverter valve, boost is pissed back into the intake after the MAF. The ECU “knows” how much air should be coming back into the intake and since its post MAF it has no active way to monitor that. So pissing all the air off to the atmosphere, when the ecu is pumping fuel in expecting it to be recirculated into the engine throws it off. Causes a rich condition.
If the BOV is recirculated (AKA should have just bought a DV) mechanically its 100% fine to do. It just not using the proper tool for the job.