Children not in HTML with Bootstrap?

Hi all,

I am using this module to attach blocks to some of the main menu links. When I am using Olivero it works fine (my child blocks are inserted into the li), but if I use this Bootstrap theme https://www.drupal.org/project/ui_suite_bootstrap → any blocks I attach as children in for example /admin/structure/menu/item/9/edit/layout are just ignored / not rendered in the html, the html just goes from one li to the next with nothing inserted.

Any ideas please? I can give link / login to my test site if that helps.