This happens because of MDL-21782 where the user is not returned by get_enrolled_sql because of the round(time(), -2) hack to utilise DB caching. The user is instantly enrolled in the course when accessing the LTI activity but the round(time(), -2) hack means the it's possible that the time used in the query is less than the current time, meaning they are not returned until X seconds has passed.

Steps to replicate

Requirements

1. Two sites.

• One for your LTI consumer.
• One for your LTI provider.

On your LTI provider site.

1. Visit 'Site administration' > 'Plugins' > 'Authentication' > 'Manage authentication'.
2. Enable 'LTI'.
3. Visit 'Site administration' > 'Plugins' > 'Enrolments' > 'Manage enrol plugins'.
4. Enable 'Shared external tool'.
5. Visit a course and create an assignment.
6. Visit 'Course administration' > 'Users' > 'Enrolment methods'.
7. Add a shared external tool with 'Tool to be provided' selected to your assignment.
8. Visit 'Course administration' > 'Shared external tools'.
9. Make note of the URL and secret.

On your LTI consumer site.

1. Visit a course.
2. Add a LTI activity and use the URL and secret from your LTI provider site.
3. Log in as a student.
4. Visit the course and click on the LTI activity.
5. Notice the assignment header is only shown.
6. Wait for 2 minutes and refresh the page.
7. Note the assignment displays as expected.