No one likes being assigned to a position below their current grade but unfortunately sometimes it is necessary. The regulation allows you to be assigned one grade down and up to two grades up, so the action is within regulatory guidelines.
Will it hurt you for promotion? The key is not the position you are in but how you perform in the job. The comments on your NCOER that reflect your performance and potential are important, and will be what the board focuses on.
Some raters provide latitude when rating a leader in this situation by placing the duty MOS at the Soldiers current grade level rather than the lower level on the MTOE. For example, if you are a 89B30 serving in an 89B20 position the rater may choose to rate you as an 89B30. This is totally his call.
AR 614-200 paragraph 3-10(10):
“Assign soldiers within the same grade or up to two grades higher if no higher-ranking soldiers are available. Utilization within the same or higher skill level should be considered. The higher skill level will include any other MOS to which the soldier would advance in the normal line of progression in the MOS career pattern…”