@JeremyECrawford It looks like monsters use their proficiency to calculate saves/skills. But there are some exceptions like Lich's Arcana
— Mark Lenser (@MLenser) April 5, 2015
A monster's save or skill bonus includes its proficiency bonus (MM, p. 8) and maybe Expertise or an ad hoc modifier. https://t.co/fRockHqbtx
— Jeremy Crawford (@JeremyECrawford) May 14, 2015