Sorry about this Zacchaeus. This one is really esoteric.
For the NPC, Illithilich, in Volo's, I believe the Intelligence Saving Throw is misspelled.
It looks correct on first inspection but I believe it is spelled lnt +12 using a lower case "L" instead of an upper case "I".
Here is a screenshot, note the "I" in Int for the Lich but the "l" (lower case "L") in lnt for the Illithilich.
Attachment 45784