Hi Theo,
I think I understand what is happening. "Jumps to the top" could mean a few things.
Hope it is something basic. You should check the jumps on the question(s). Sounds like the student selected an answer which had "This page" set as the jump. The student would go back to the page and have to scroll down to answer the question again. There should be at least one answer that sends the student to the "Next page" or some other place in the lesson.
One of the places you could send them is the place called "End of Lesson". When a student jumps there, they have finished the lesson and it is graded right then.
Several options to make jumps work they way your want ("disable jumps"). 1) In Lesson settings, change maximum attempts to 1. With this settings, when a student misses the question and the jump says "this page" they will go to the next page. If you set it to 2, then when the student misses the question not once but twice, they will go to the next page on their second miss. Option 2) This is more complicated and more work. You can turn on custom scoring, then make sure every answer goes to the next page. Give a score for the correct answer and 0 (zero) for all other answers.
You did not tell us if scoring the question was important to you. My assumption is that you want to score an answer. With custom scoring turned off in Lesson settings, any answer's jump that goes towards the last page, is counted as a correct answer. Any answer that goes to "This page" or one towards the first page is counted as wrong.
There is a lot of information in
MoodleDocs that might explain this differently. I would suggest starting with
Lesson pages and pay attention to the
sections about "Jumps in general" and "Jumps in questions". There is a page called
Jumps which gets into the difference between different types of jumps, that is not your issue.
Hope this helps a little. Chris