This printing defect may be caused by damage to the Brother toner cartridge. This is most often due to mechanical wear on the metering bar. A large amount of toner adheres to the transfer roller, and the excess falls into the drum cartridge and then onto the paper.
We recommend replacing the toner cartridge to avoid heavy dirt on the printer. And in the case of prolonged use of a cartridge with such a defect, the printer or mfp should be prevented. Because even if you install a new cartridge, the accumulated toner inside your Brother printer will continue to stain the paper.