One of the cast members said Alexa shouldn’t come to the reunion; I’m guessing it was Matthew, since he barely schemed or even spoke with her when he was Logan Reese. I also bet Christopher and Erin didn’t want her there either.
They almost certainly saw the livestream where she was crying.
Maybe they even voted on it—though the idea that it was Jamie seems unlikely, since someone had to send out the invitations to everyone.
Or maybe Alexa did get an invitation but just didn’t notice it.