Yes I’m reffering the Blackburn theme on GH and I’m using Your menu configuration for the test
My answer is only base on assumption. in this case It will be hard find the real answer if you did not include a complete information or your project repo, that’s why @maiki ask you to read Requesting Help
This probably caused by the CSS (overflow: hidden or else) so you cant see anything when you click the parent menu
this can be a clue (again only base on assumption)
For better answer consider to contact the theme’s author