TYPO3 Core

FlexForm configuration to reproduce the issue:

<?xml version="1.0" encoding="UTF-8"?>
<T3DataStructure>
    <meta>
        <langDisable>1</langDisable>
    </meta>
    <ROOT>
        <type>array</type>
        <el>
            <linklist>
                <type>array</type>
                <section>1</section>
                <el>
                    <item>
                        <title>LLL:EXT:acme/Resources/Private/Language/locallang_db.xlf:linklist</title>
                        <type>array</type>
                        <el>
                          <linkitem_link>
                              <TCEforms>
                                  <label>LLL:EXT:acme/Resources/Private/Language/locallang_db.xlf:linkitem_link</label>
                                  <config>
                                      <type>input</type>
                                      <renderType>inputLink</renderType>
                                      <size>30</size>
                                      <eval>trim,required</eval>
                                      <fieldControl>
                                          <linkPopup>
                                              <renderType>linkPopup</renderType>
                                          </linkPopup>
                                      </fieldControl>
                                  </config>
                              </TCEforms>
                          </linkitem_link>
                        </el>
                    </item>
                </el>
            </linklist>
        </el>
    </ROOT>
</T3DataStructure>