It was not before long that Bigod became agitated under the rule of law initiated by Henry.

https://en.wikipedia.org/wiki/Hugh_Bigod,_1st...

However, it was not before long that she was reminded of her limitations in the most unexpected of ways,

https://nationalinterest.org/blog/buzz/why-i-...

Some more - published - sources for "not before long":

John Fobanjong (Prof., UMass), Understanding the Backlash Against Affirmative Action

Not before long, however, affirmative action's "golden age" would gradually come to an end with the retirement of Justice Warren in 1969 and the appointment of Justice Burger as the new Chief Justice of the U.S. Supreme Court.

Laurel Brake (Prof. Emerita, Birkbeck College, University of London), The Endings of Epochs

Not before long was Decadence condemned in nearly every quarter of the literary establishment.

Aviva Drescher, Leggy Blonde: A Memoir

Her facade would crack, though, and not before long. “When you came out of my womb, the first thing I saw were your long, long legs,” she used to say when telling the story of my birth.

(For the record, the second example was written not by Laurel Brake but by one Joseph Bristow, a professor at UCLA, in an essay contained within the volume. But that makes no difference.)

It seems to me that "Not before long" is a careless amalgamation of "It was not long before ..." and "Before long". Whether or not you designate it as wrong, it seems rather silly: "Not before long was Decadence condemned ..." is used to mean precisely the opposite: "Before long, Decadence was condemned ... ."

John Fobanjong uses "not before long" at least four times, and in each instance "before long" would have been perfectly adequate.