TYPO3 Core

ext:irre_tutorial ships with a setup to test inline relations with foreign_field definition. This setup is bound to a tt_content field on the one hand, and as a flexform definition on the other hand.
The flexform configuration has a foreign_match_fields definition applied, while tt_content setup has not. This leads to flexform references being applied to tt_content field, too, but not vice versa.
So when a flexform relation existed, it was valid for the tt_content fields as well.

Especially in test setups, explicit, unique and valid configuration is imperative to provide stable and reliable tests. To resolve this implicit relation recyling, the tt_content configuration requires a foreign_match_field definition with a different value.