TYPO3 Core

The section menu CE configured to type 3 (section index) takes into account content elements from colPos == 0.
This is because the viewhelper typo3/sysext/fluid_styled_content/Classes/ViewHelpers/Menu/SectionViewHelper.php

sets the default value for argument "column" to 0 (empty would mean to take into account CE's from all columns).

So currently if you use different colPoses than 0 section index will not show any link.

There are 2 solutions:
1. change default value in the viewhelper
2. change template to pass empty column in
typo3/sysext/fluid_styled_content/Resources/Private/Partials/Menu/Type-3.html

<ce:menu.section pageUid="{page.uid}" as="contentElements" type="header">

<ce:menu.section pageUid="{page.uid}" as="contentElements" type="header" column="">

tested in 7.6.9