It would seem that Starfleet knew the E-C had been present to defend Narendra III, but that there were no Fed or Klingon spatial survivors to tell the story of how that defense actually went. So Starfleet in the regular timeline would have no leads to go by, no reason to assume anybody from the ship survived. The result of the investigation to the loss of the ship would be "Blasted to bits by Romulans without survivors or cooperative eyewitnesses", end of story.
In the war timeline, we might assume that there were some Klingon witnesses to the "cowardly retreat" of the E-C, and that they told Starfleet nothing but went on a war footing and eventually attacked. Or at least the planetside Klingons knew the E-C had come to protect them, and then the Romulans came and won, and the Klingons afterward were able to ascertain that there was no wreckage of the Starfleet vessel anywhere -> cowardice!
Rumors of Federation survivors? Since when did rumors travel out of the isolationist RSE?