TYPO3 Core

Good one! You could provide a patch if you'd like to... ;-)

Ugly patch attached.

It looks like some more magic is needed since the user may specify other tt_content fields too:

<div id="content-element-{field:uid}-{field:crdate}-blah">

But it fixes the simplest case where only uid is used.

Still present in 6.2 I guess:

/typo3/sysext/core/Classes/TypoScript/TemplateService.php, line 1529:

$LD['sectionIndex'] = $page['sectionIndex_uid'] ? '#c' . $page['sectionIndex_uid'] : '';

$sectionMark = $sectionMark ? (\TYPO3\CMS\Core\Utility\MathUtility::canBeInterpretedAsInteger($sectionMark) ? '#c' : '#') . $sectionMark : '';

$sectionMark = (\TYPO3\CMS\Core\Utility\MathUtility::canBeInterpretedAsInteger($sectionMark) ? '#c' : '#') . $sectionMark;

The string " '#c' " is also present in:
/typo3/sysext/linkvalidator/Classes/LinkAnalyzer.php
/typo3/sysext/linkvalidator/Classes/LinkType/InternalLinktype.php

I made a patch, though I'm not sure this should even be solved. It's an edge case.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/57321

To test this behaviour following typoscript setup will help.
Use this setup as root typoscript template on any page with some content elements of any kind (header needed).

/**
* TypoScript setup to test issue #18932
*/

// New config variable 'sectionLinkPrefix' - you need to adapt content wraps
// (typo3/sysext/fluid_styled_content/Resources/Private/Layouts/Default.html)
config.sectionLinkPrefix = section-element

// Default menu - with section index enabled
// (https://docs.typo3.org/typo3cms/TyposcriptReference/MenuObjects/Sectionindex/Index.html)
lib.defaultMenu = HMENU
lib.defaultMenu {
    entryLevel = 0
    1 = TMENU
    1.NO = 1
    1.NO.allWrap = <li>|</li>
    1.wrap = <ul>|</ul>
    1.sectionIndex = 1
    1.sectionIndex.type = all
    2 = TMENU
    2.NO = 1
    2.NO.allWrap = <li>|</li>
    2.wrap = <ul>|</ul>
    2.sectionIndex = 1
    2.sectionIndex.type = all
    stdWrap.outerWrap = <h2>Default menu - HMNEU with section index enabled</h2>|<hr />
}

// Section Menu - typolink with section in a CONTENT obj rendering context (integer)
lib.sectionMenuInteger = CONTENT
lib.sectionMenuInteger {
    table = tt_content
    renderObj = TEXT
    renderObj {
        field = header
        typolink {
            parameter.data = TSFE:id
            section.field = uid
            wrap = <li>|</li>
        }
    }
    wrap = <ul>|</ul>
    stdWrap.outerWrap = <h2>Section Menu - typolink with section in a CONTENT obj rendering context, where section field is integer (uid)</h2>|<hr />
}

// Section Menu - typolink with section in a CONTENT obj rendering context (string)
lib.sectionMenuString = CONTENT
lib.sectionMenuString {
    table = tt_content
    renderObj = TEXT
    renderObj {
        field = header
        typolink {
            parameter.data = TSFE:id
            section.field = header
            wrap = <li>|</li>
        }
    }
    wrap = <ul>|</ul>
    stdWrap.outerWrap = <h2>Section Menu - typolink with section in a CONTENT obj rendering context, where section field is string (header)</h2>|<hr />
}

// Section link - simple typolink with section
lib.sectionLink = TEXT
lib.sectionLink {
    value = Link to this page
    typolink {
        parameter.data = TSFE:id
        section.field = uid
    }
    wrap = <h2>Section link - simple typolink with section</h2>|<hr />
}

// Page object - collects all examples above
page = PAGE
// Default menu
page.10 < lib.defaultMenu
// Section menu
page.20 < lib.sectionMenuInteger
page.30 < lib.sectionMenuString
// Section link
page.40 < lib.sectionLink
// Content
page.100 < styles.content.get

Searched for "'#c'" string on latest master(10.4.0-dev) and found it in these files:

/typo3/sysext/backend/Classes/ContextMenu/ItemProviders/RecordProvider.php

/typo3/sysext/backend/Classes/Controller/EditDocumentController.php

/typo3/sysext/linkvalidator/Classes/LinkAnalyzer.php

/typo3/sysext/linkvalidator/Classes/Linktype/InternalLinktype.php

/typo3/sysext/recordlist/Classes/RecordList/DatabaseRecordList.php