Incognito breaks select2?

I am having a problem with incognito mode, when not in incognito mode the select box loads fine.
However if I open the page in incognito mode it looks like this:
Screenshot from 2018-07-19 12-11-04

I have cleared my cache and that doesn’t seem to make a difference!
It looks to me as though the aria-expanded and the aria-owns tags being missing from the broken version is the main problem however I’m not sure what the differences is that is causing the to not get added?
any help or ideas on this would be much appreciated!
Thanks a lot,

I can’t reproduce this behavior with my own Select2. Can you post your code in a jsfiddle or codepen?

I’m guessing this problem is due to something specific about your site or web page, but without seeing your code I can’t troubleshoot it any further?