That's Brandon's style of storytelling through the book(s). He doesn't foreshadows or sets things up for later use nearly as much as Jordan did (he constrain himself to setting up future scenes from the current book, pretty much. He doesn't scatter around details that will return to be important much later, the way Jordan used to), and he doesn't follow through nearly so much on minor plot points (just think for instance that Elayne told Mat at dinner he would have to tell her everything about his wedding... and this never got mentionned again. We also have zero reaction or thoughts from Elayne about the Seanchan attack on Tar Valon, on the risk they sole travelling! As far as we know, she never even discussed the attack, the strategy of the Seanchan, their strength etc. with Gawyn. But for certain, Elayne knows about the attack, that Gawyn was in the middle of it, that Elaida got captured and so on. If you think omitting her thoughts on Duhara post-purge is "weird" given her interest in the topic, what about those omissions...) The return of Duhara in the plot is probably planned only for AMOL, so Brandon kept Elayne's further thoughts on Duhara and the BA for AMOL. Once Egwene became Amyrlin, Duhara would stop being a top priority for Elayne, Black or not. She already has spies around her. She's small fish compared to the issues Elayne was dealing with in her post-meeting-with-Egwene scenes. It's not only the issue of Duhara Elayne has not thought about or brought up in the scenes after she's met Egwene anyway, it's the whole Black Ajah purge.

I'm not really satisfied with the way Brandon handled all this and all the limitations and ackwardness caused by his choice to present the scenes out of chronological order, but it's how it is. The scene with Egwene makes clear she and Elayne brought each other up to speed at length, and as Egwene is perfectly aware a large number of sisters are in Caemlyn, it makes no sense she's not warned Elayne about all the missing BA and suspected BA, the very same way she's warned the BT envoys (which could have some big plot relevance, if the BA hide at the BT).