Game concept
Rightful Liege
From the game's encyclopedia
The Rightful Liege of a Vassal is the Ruler who holds the direct De Jure Liege Title of the Vassal's Primary Title. If a Vassal is not under their Rightful Liege, they will contribute less Taxes and Levies as well as suffer an Opinion penalty towards their Liege. Holding a higher-Tier Title of the same De Jure results in lessened penalties.
An exception to this is Dukes that are part of an Empire's De Jure hierarchy and directly vassalized to their Emperor, which still contribute fully, exactly as if the Emperor held their De Jure Kingdom Title.
Example: The Count of Dunbar is a direct vassal of the King of England. Since the King is neither Duke of Lothian, King of Scotland, nor Emperor of Britannia, the Count of Dunbar only contributes a tiny amount of their taxes and levies.
The Count of Norfolk is also a direct vassal of the King of England. The King does not hold the Duchy of East Anglia, so the Count also contributes reduced taxes and levies. However, because the England is the De Jure Kingdom of Norfolk, the Count of Norfolk still contributes significantly more tax and levies to the King than the Count of Dunbar does.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Rightful Liege is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Rightful Liege of a Vassal is the Ruler who holds the direct De Jure Liege Title of the Vassal's Primary Title." It is part of Liege. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.