They must have heard about it as soon as Kushner amended his security clearance forms. The exact date he did so is not clear but the Times reported in their intiial report on this meeting that they did so "recently", which would coincidence with this event. In addition, Manafort disclosed the meeting recently to congressional investigators behind closed doors.

It’s unclear exactly when Kushner submitted supplemental information, or whether he did so all at once. As of early April, when some of Kushner’s other previously undisclosed meetings with Russian nationals first surfaced, his team said he was still working on gathering all the necessary information. Both the CNN and Times reported that the disclosure of the meeting with Veselnitskaya occurred “recently,” immediately after Kushner’s legal team discovered the email chain while preparing for testimony before Congress.

The emails were discovered in recent weeks by Mr. Kushner’s legal team as it reviewed documents, and the team amended his clearance forms to disclose it, according to people briefed on the developments, who like others declined to be identified because of the sensitive political and legal issues involved.

Similarly, Mr. Manafort recently mentioned the meeting to congressional investigators looking into possible collusion, according to the people briefed on the matter.

[NY Times] 7/11/2017