Commander is reported to have chomped down on another Secret Service agent Monday ... this according to a spokesperson for the agency. They confirmed that one of their guys had been bitten around 8 PM and that he was treated by medical personnel.

The exact circumstances of what led up to the biting and the extent of the SS agent's injuries haven't been disclosed -- but regardless ... it's proven to be a troubling trend this year.

Remember, it was revealed in July that Commander apparently went on a 4-month biting spree ... attacking several different Secret Service agents with varying degrees of wounds. The number we'd heard at the time was 7 ... but some reports had it as high as 10.

The White House Press Secretary said the dog was stressed out by all the commotion and people darting back and forth in the residence area of The White House and other areas as well.

When this came to light in the summer, the White House said they were going to have Commander undergo more training/leashing ... and that they'd also carve out designated areas and times on the grounds for Commander to roam and stretch his legs.

There was no indication that Commander would get the boot like his older brother, Major ... another German Shepherd of Biden's that had also run into biting trouble.