TYPO3 Core

I'm having (new) issues with section navigation using latest HEAD. I'm using this TS:

lib.section_index =< tt_content.menu.20.3
lib.section_index {
    stdWrap {
        wrap = <ul class="nav bs-sidenav" >|</ul>
        outerWrap = <div data-spy="affix" data-offset-top="210" class="bs-sidebar hidden-print" role="complementary">|</div>
    }
    1 {
        NO.ATagParams.dataWrap = data-target="#c{field:sectionIndex_uid}" 
        NO.wrapItemAndSub = <li>|</li>
    }
}

All works fine, except 'sectionIndex_uid', which always stays empty. Moving back to 6.2.0beta3 brings back the field and it's content. Couldn't find any changes in TSref, so this might be an issue.