The NWT translates the Greek words "ego eimi" as "I am" every time it appears (Jn 6:34, 6:41, 8:24, 13:19, 15:5, etc.), except in Jn 8:58 where it is translated as "I have been". What is the reason for the inconsistency in this translation? If "ego eimi" was translated in Jn 8:58 the same way it is translated in every other verse in which it appears, how would Jn 8:58 read? See Exo 3:14.
If ego eimi is transliterated into English with no consideration for the context and intended meaning then you would have a nonsensical expression, as is the case with most translations that attempt to make a proper noun out of the predicate, ego eimi. It demonstrates the power of brainwashing, that otherwise literate people can read John 8:58 in popular versions and pretend that the verse actually makes sense. In reality, popular versions of John 8:58 are grammatically incorrect. It is amusing that trinitarians are so desperate to make Jesus out to be God that they insist on using fraudulent translations of the Scriptures.