TYPO3 Core

https://github.com/TYPO3/typo3/blob/main/typo3/sysext/indexed_search/Classes/Controller/SearchController.php#L147-L155¶

The source location above contains a hint concerning the rl0 field, that is used and filled during the indexing process, based on sys_template records having the root flag set (which is not the case anymore when using Sets).

https://github.com/TYPO3/typo3/blob/main/typo3/sysext/indexed_search/Classes/Domain/Repository/IndexSearchRepository.php#L722-L726¶

The source location above adds a constraint like rl0 IN ('0') (also due to "magic" type-casts of configuration property rootPidList) - which results in not showing any results (even if data has been indexed).

https://github.com/TYPO3/typo3/blob/main/typo3/sysext/indexed_search/Classes/Indexer.php#L1424¶

The source location above actually sets & persists the rl0 value during the indexing process, based on sys_template records having the root flag set (which is not the case anymore when using Sets).