Issue with web browsers not recognising a course has been completed

This is the first time I had logged a question on the forum so apologies in advance if my query has been raised by someone else. 

Has anyone come across an issue whereby web browsers are not recognising that a course has been completed and so users see a browser pop up ‘leave site? changes that you make may not be saved’ after the course has been marked complete on an LMS?

I have a course that has been created in Captivate version 12.0.4.20 and published locally with enabled reporting settings for xAPI, success settings 100% screen views. 

We have a Learndash LMS with Grassblade plug in. The Mark completion button is working as it should on the LMS, course tracking is recording as it should when eLearning has been viewed, but when the user clicks the link to go to the quiz which is associated with the course, the web browser prompt comes up. 

The user can click to leave the site and it take them to the quiz to complete, but clearly I don’t want learners getting confused by the browser prompt as most are likely to cancel it. 

Any guidance on settings or anything related would be really appreciated as I have been trying to find a solution for over 10 hours!!

thank you