Hi !
Exact same problem here : the div containoing the link to add a new related entity is displayed in front of the select, so that clicks don't go to the select...
Hiding this block using CSS fixes the problem, but hides the add link too...
Oddly, it seems that it's the javascript block which causes this problem. When I move the javascript block in the DOM before the select or after the add link block, it displays correctly.
I'll see if it's easy to fix in the code and make a PR if so.
Best,