TYPO3 Core

Some findings, also shared on slack where this was reported in #typo3-cms:

That's odd. vendor/typo3/cms-core/Classes/TypoScript/IncludeTree/SysTemplateTreeBuilder.php in method getBasedOnSysTemplateRowsFromDatabase calls:

$queryBuilder->setRestrictions($this->getSysTemplateQueryRestrictionContainer());

and that uses:

        
$restrictionContainer = GeneralUtility::makeInstance(DefaultRestrictionContainer::class);
if ($this->context->getPropertyFromAspect('visibility', 'includeHiddenContent', false)) {
    $restrictionContainer->removeByType(HiddenRestriction::class);
}

so theoretically it should include the basedOn records only if the context property includeHiddenContent is enabled. Any chance you set that somewhere?

Or do you maybe use the frontend admin simulator toolbar and have that property enabled?

We have finally gotten to the bottom of this and it's not a TYPO3 core issue. What we did was, we disabled TypoScript records in an Update script, using DataHandler and forgot to add the cache clearing in the script. So the TypoScript record was disabled in the backend, but effective in the frontend - until the frontend cache was cleared.