She also learned of the invasion plans, but she mysteriously (and willingly) chose not to reveal this to her inner circle for now. It sounds like a convenient plot device, either because the story needed for Elayne to have made a big mistake (in the sense that simply having her never learn about the invasion beforehand wasn't an option, the fact she knew and didn't act will become a relevant story point) or to avoid a plot hole by explaining Elayne has not forgotten but chose to defer the issue to later discussion, and thus hide completely whatever measures she may have put in place behind the scenes regarding this attack, before she left Caemlyn.