MS Form autograding has bugs?

Hi all. I got two queries.

1) I did a quiz for students in my class. I noticed that the answer for a question which is 75393, which is graded as correct for all students except for that one particular student (check the pics below). I don't know why and what when wrong even though the student answered it in the exact correct answer. 

2) As a result of the query above, I decided to self-grade the students, however I noticed that I couldn't tick it as correct or have it shaded in green - which would mean correct. Is there a way to solve this?