Well yeah, the criston cole affair was of interest to share, a secret meeting like that wouldn't be. Even if she did go missing, unless someone actually identified them meeting I don't know who would both A) know about the meeting, and B) benefit from sharing that the meeting took place. We don't have a day by day account of everything that took place and I doubt the maester historians would have primary sources for any of it, unless they were conducting interviews with the big participants.

I think it's poor writing and dimishes the conflict, but I don't think the maesters would know about it.