"Drag and drop into text with unlimited options" does not show the option after navigating quiz

When I choose multiple option for the drag and drop text question and then navigating to other quiz, it seems that the options are disappeared when I choose back the original one.

Here are the steps in order to reproduce.

1. Create a quiz activity
2. Add a question of Drag and drop into text with unlimited option

       ex:

select.     [[1]]             [[1]]
select.     [[2]]             [[2]]
select.     [[3]]             [[3]]

     3. Add a second question.For example multiple choice

     4. Click preview to start a quiz

    5. Drag any number into the empty slot for 1st question

    6. Click quiz navigation to go to 2nd question

    7. Click 1st question again to go back

    8. Selected option are disappeared from the selection area

Intermittently, I could not drag any items  when go back to the drag and drop questions.