'Open URL in new window' trigger works but then web page immediately closes

I have tried everything I know how to do so thought I'd try and source a solution here. 

I have a Storyline 360 file on my local drive. I am running the current (up-to-date) version of Storyline.

The course is an incredibly basic course with a 'click and read style' rather than a highly interactive piece. There are no variables and navigation is done via triggers. Buttons are simple and either take learners to a layer or to URLs outside the course. 

On 2 different pages there are buttons that should take the learner to an external URL. The trigger is set to open the URL in a new window. The website is correct and opens fine and remains open when input directly into a browser. Through the button on Storyline it opens the webpage and then the page immediately closes. Other buttons that take learners to other URLs do not have this issue. The settings are the same on all buttons.

I have tried deleting and recreating the button and all solutions from this old post.

I have tried:

• creating a new file and importing content 
• checking naming conventions and 'best practice' when saving
• uninstalling and reinstalling Storyline to repair the issue.

Has anyone else had this problem? If yes, did you find a solution?

Thanks in advance for any help.