The phrase "curious omission" means that something important or expected was left out or not included, and it is unusual or interesting that it was excluded. Full definition
That didn't pan out so well, with many customers preferring other flagships thanks to the OnePlus 2's curious omission of NFC and other features. (9to5google.com)
PROOF that this is right - wing BS... No one complained about making viagra available to priests... hmmmmmm curious omission?? (religion.blogs.cnn.com)
Though we already know that Ian McKellan, Patrick Stewart, Hugh Jackman, Ellen Page, Shawn Ashmore, and Anna Paquin are set to reprise their X-Men characters, one curious omission thus far has been Halle Berry. (collider.com)