Hello
We are on Moodle 3.9.3.
With stack questions, we can see two different behaviors :
For example, this is the student input : 3^n
- With deferred feedback, stack is not working : here is the output
- With immediate feedback, everything works fine. 
Are you facing the same issue ?
Do I need to post it on the stack tracker ?
Question.xml is in attachment, if you want to try.
Thank you.