Steps to reproduce the issue

• Joomla 3.8.5 or current staging.

• Create menu structure with 3 or 4 levels. In my example/images menu name is "Main Menu-EN"

-  Level 1
- - Level 2
- - - Level3
- - - - Level 4

• Create a module of type mod_menu. Settings:
  

• Create another module of type mod_menu. Settings:
  

• Don't forget position and menu association "On all pages"

• Go to frontend and open menu item "Level 1"

Expected result

Both modules with split menus of level 2 and level 3 are shown.

Actual result

Only level 2 menu is displayed. For level 3 menu ModMenuHelper returns an empty list.
Because of line https://github.com/joomla/joomla-cms/blob/3.8.5/modules/mod_menu/helper.php#L69

Additional comments

Is this really the expected behavior? If yes, I don't understand why.