While an instructor can develop their own solution to submit for checking, or even look up a student's submission and submit it for checking, these submissions are only allowed before the assignment deadline. After the deadline passes, instructors cannot submit anything (just like students). Instead, instructor accounts (even when viewing a student repository) should be able to submit work for grading and get results, even after the deadline passes.
While an instructor can develop their own solution to submit for checking, or even look up a student's submission and submit it for checking, these submissions are only allowed before the assignment deadline. After the deadline passes, instructors cannot submit anything (just like students). Instead, instructor accounts (even when viewing a student repository) should be able to submit work for grading and get results, even after the deadline passes.