Spells Not Showing In NPC Sheet

I was preparing the final fight of LMOP and i decided i wanted to alter the final battle. The problem was that when i switched out the final boss for another, the spells for the npc i placed in did not appear.This was consistent for both Mordekainens and Monster Manual. Are there any solutions? Anything i can do? or is it a known bug? The spellscasting NPCs that came with LMOP had their spelllist

Spellcasters in the compendium by default are not populated with their known spells. This has been a limitation since the monster manual was added to the compendium. The only solution you have is to drag each spell from the compendium to the character sheet for the spellcaster.

................. Thanks for the Answer!