Clicking on enrol users buttons before javascript loads leads to the manual enrolment screen which confuses users

What is supposed to happen:
We are trying to enroll users in a course, so we go to the course, click on Users under Course Administration and click Enrolled Users. It takes us to the /enrol/users.php?id=8. From there we click the blue button titled Enroll Users and a popup window appears where we can search for the users and add them manually.

What intermittently happens:
When enrolling users into a course, if you click the "Enroll Users", it does not load the enrollment plugin in a popup window, it opens a new page.

How to duplicate the issue:
It seems to be an issue with the speed at which the page loads. If the page doesn't load completely or if you refresh the page and immediately click "Enroll Users", it takes you to /enrol/manual/manage.php?enrolid=19&id=8 instead of sideloading in a popup window.