The Favorable Beginnings Illithid power which is supposed to apply a proficiency bonus whenever an ability check is used against an NPC unfortunately does not work when the proficiency in question is obtained as a Background Proficiency.

In the example below the Dark Urge character has Intimidation (gained through Background) and Persuasion (chosen proficiency), but Intimidation does not benefit from the power precisely because it's bugged for background proficiencies.

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[Linked Image from bg3.wiki] Minthara's Compendium Of Issues