The only thing Disco said about when reunification happened was that it was "centuries ago" but also "centuries after Spock's death." With that in mind, my head canon is that it occurred around the 27th or 28th century. So, at that point we're talking about three to four hundred years later, unless one of them were a Lanthanite, I doubt anyone currently alive cared very much about the fact that an ancestor who would have been dead by now anyway actually lived for a few years after their recorded death as a prisoner of war before they actually died.
And all that's assuming records of Enterprise C survivors were even still available when reunification occurred. We are talking about an empire which collapsed due to the destruction of its homeworld and subsequent fracturing of its territory into many smaller nations. By the time of reunification, it's possible no Romulans alive had any knowledge of Starfleet officers being taken prisoner in the mid 24th century, nor were any records on the matter still available. The accounting on this matter would be the story Sela told Picard, which I guess can be verified by Sela's own existence.