TSFE getPagesTSconfig does not use included registered page TSconfig
Registered Page TSconfig files can be included in arbitrary pages by selecting the items in the field "Include Page TSConfig (from extensions):". In the function \TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController::getPagesTSconfig() these includes are not taken into consideration.
In the corresponding BE function \TYPO3\CMS\Backend\Utility\BackendUtility::getPagesTSconfig() the includes are included.
If you include a registered TSconfig file with linkhandler configuration the new tab appears in the backend, but the link won't be rendered in the frontend because the TSconfig is missing.