https://screenrant.com/georgie-and-mandys-first-marriage-episode-16-pastor-jeff-didnt-appear-in-the-big-bang-theory-reason-explainer/

While Georgie and Mandy fight, First Marriage (https://screenrant.com/db/tv-show/georgie-mandy-s-first-marriage/) episode 16 explains why 1 prominent Young Sheldon character never appeared in The Big Bang Theory.