The new New World Translation translates Jeremiah 29:10 as follows:
10 “For this is what Jehovah says, ‘When 70 years at Babylon are fulfilled, I will turn my attention to you, and I will make good my promise by bringing you back to this place.’
The official Watch Tower Society teaching is that by the end of the 70 years, the Jews were back in Judea, and that their actual arrival in Judea marks the end of the "70 years". For example:
*** si p. 85 par. 3 Bible Book Number 15—Ezra ***
A faithful remnant journeyed back to Jerusalem in time to set up the altar and offer the first sacrifices in “the seventh month” (Tishri, corresponding to September-October) of the year 537 B.C.E.—70 years to the month after Judah and Jerusalem’s desolation by Nebuchadnezzar.
*** it-1 p. 463 Chronology ***
Hence the count of the 70 years of desolation must have begun about October 1, 607 B.C.E., ending in 537 B.C.E. It was in the seventh month of this latter year that the first repatriated Jews arrived back in Judah, exactly 70 years from the start of the full desolation of the land.
*** w11 12/15 p. 31 Do You Remember? ***
The Jews were released, and they arrived in their homeland by 537 B.C.E. The Bible says that their exile was 70 years long. So Jerusalem must have fallen in 607 B.C.E.
That being the case, if the Jews were already home when the 70 years ended, in what manner would Jehovah "make good my promise by bringing you back to this place" after they were already there?