Why would she be missing from the other photo? I agree the two photos shown are not the same, but it still appears she was cropped from the top photo unless she was in the ladies room. You’d need to see the video to check the timeframe.
She was indeed cropped out - look at how tightly Mattis was cropped on the left side to avoid even showing the shadow of the woman. Wen you look at the video of the event, she is walking onto the stage with the other attendees. She, by the way, is Kistjen Nielsen, the Homeland Security Secretary and only 45 years old, not just some nobody.