I have two editions of the Greek New Testament--one omits the "Amen" entirely while the other prints it in brackets. The ancient manuscripts and the quotations in the Church Fathers are divided but it appears that more omit the "Amen" than include it. My Latin New Testament includes it.
Not a big deal--it doesn't change the meaning of the text, but it is an example of how we are dependent on the scribes who hand-copied manuscripts. I think it is easier to see a scribe unthinkingly adding the "Amen" just because it so often punctuates an emphatic statement, than that the other scribes would have omitted a word that was in the text.
The “Amen” was read as part of the Second Reading this morning at Mass.