submenu links 4th level not displayed w Picture Reloaded

I have a problem with main navigation submenu links (4th level) not displaying in a Drupal 8 test site, with Picture Reloaded theme (main navigation menu, 5th link “Η περιοχή της Οχης”, 2nd submenu link “Η φύση της Όχης”, the 1st and 6th submenu links, ie, “Φαράγγια” and “Μονοπάτια” have children submenu links that are not displayed).

All settings are correct (related block menu levels is set to unlimited), all parent menus are set to expanded, parent links are correctly set.

When Adaptive Themes becomes the default theme, the submenu links are displayed correctly, but in Pixture Reloaded, they are rendered in the markup, but they are invisible.

I guess this is a bug, as I have read similar issues in other forum posts.

Appreciate any tips.

Drupal version: