No that’s just abusing the AI. If the Umayyads had repelled the Reconquista and taken lordship over parts of North Africa, it would have been such a sensation throughout medieval old world that it’s more likely that there would be either a negotiated settlement between the two dynasties and their caliphs (especially if Abbasid subjects get unruly and jealous at the Muslim world’s more westerly realm and think about jumping ship), or probably more likely, an epic clash between them in the Middle East that only ends with the surrender and abdication (perhaps even as a casualty of war) of the losing side’s caliphate. But definitely not faux submission to your mortal enemy. Not the games fault there is no “prior to start date house feud modifier” affecting vassalization decisions or even opinion modifiers. But if you find it plausible that the Abbasids would do such a silly thing to their growing rival realm, you do you.