Menu expands at first page load then collapses when page load finished

Hope somebody can help. I’ve nearly finished a website but noticed it seems to have an unfortunate bug in this theme where the menu is fully expanded (showing all levels) when loading, then back to normal (collapsed) when the page load finishes.

Can anybody please help in narrowing this down to what’s causing it and how I can fix it? I’m not sure exactly what’s causing this behaviour, nor do I know how to fix it yet. I’ve tried troubleshooting it lightly but nothing worked yet.

Thank you in advance.

Reference website: http://www.glovermedical.com