When a person believes in a jot and tittle accurate Bible, they believe that every spelling at every place is correct. If the Bible says “Immanuel” at one place (Isaiah 7:14), that cannot be substituted with something else, even though at another place we find “Emmanuel” (Matthew 1:23).
This is not an inconsistency at all. The reason for two different spellings is because in the Old Testament, it was written in Hebrew, and so that transliterates into English differently than going from Greek, in the New Testament, to English.
For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. (Matthew 5:18).
Every place must be fully accurate, after all, it is God’s words we are dealing with in English, not just anyone’s subjective opinion.