Parsons problem distractors do not show as visible errors when last block

If a distractor in a Parsons problem is in any position BUT the last position, it turns red and shows as a visible error when checked. However, if the distractor is at the end of the solution, nothing is done to highlight it when checked. The problem does not even indicate that the solution is incorrect - just does not respond.

Sample problem. To see error, place blocks in shown order and hit check. Then move x + 1 up a slot or two and check

.. parsonsprob:: sample
    :numbered: left
    :adaptive:

    Add 1 to x and print.
    -----
    x = 10
    =====
    x = x + 1
    ===== 
    print(x)
    ===== 
    x + 1 #distractor

Start by reproducing the issue with the provided parsonsprob:: sample, placing the distractor in the final position and checking the solution. Trace the Parsons checking and error-highlighting entry point to compare this case with distractors in earlier positions. Done means a final-position distractor is visibly marked as an error and the solution indicates that it is incorrect.