Pretty candid discussion in their main publication on the topic The Divine Name that will Endure Forever:
There have only been a few occasions when they write candidly that there are no known fragments with the divine name.
Look how evasive they are in Reasoning from the Scriptures, not at all mentioning it has never been found, even though that is what they are answering. It is like a game of Jeopardy, "here is the answer, guess what the real question was?"
*** rs p. 278 New World Translation ***
Why is the name Jehovah used in the Christian Greek Scriptures?
It should be noted that the New World Translation is not the only Bible that does this. The divine name appears in translations of the Christian Greek Scriptures into Hebrew, in passages where quotations are made directly from the inspired Hebrew Scriptures. The Emphatic Diaglott (1864) contains the name Jehovah 18 times. Versions of the Christian Greek Scriptures in at least 38 other languages also use a vernacular form of the divine name.
It is similar in the Watchtower 2008. The whole article is about why they added Jehovah to the New Testament, but not once do they say it has never been found. It rather introduces the topic with a more evasive comment:
*** w08 8/1 p. 18 Should the Name Jehovah Appear in the New Testament? ***
"Bible scholars acknowledge that God’s personal name appears in the Old Testament, or Hebrew Scriptures. However, many feel that it did not appear in the original Greek manuscripts of the so-called New Testament."
JWs are not likely to remember or even noticed the candid quote from 3 decades ago, what JW's remember are the numerous times Watchtower claims to have rightly restored the word Jehovah to the Bible, without any information that it was done despite contradicting all known New Testament fragments.
Credit is given to Jehovah protecting his word. You cannot have it both ways, did he protect it, or was the Divine name in the NT?
*** it-1 p. 313 Bible ***
All credit and thanksgiving for the Bible’s survival in view of such violent opposition is due Jehovah, the Preserver of his Word. This fact gives added meaning to the apostle Peter’s quotation from the prophet Isaiah: “All flesh is like grass, and all its glory is like a blossom of grass; the grass becomes withered, and the flower falls off, but the saying of Jehovah endures forever.”
*** it-1 p. 1206 Inspiration ***
God’s own purpose in preparing the Sacred Scriptures and the inspired declaration that “the saying of Jehovah endures forever” give assurance that Jehovah God has preserved the internal integrity of the Scriptures through the centuries.