Hello!
Thank you for making a bug report!
Based on the information provided, Machina Citadel is working as intended. The text on Citadel destories cards the opponent controls with attack less than the monster destroyed. However, face-down cards are unknown to the gamestate. Omega reminds you what the card is if you’ve seen it before it was set, or if it was validated, however, in real play, you would not be able to continuously validate the face-down cards. Additionally, Face-down cards are only validated as monsters, however do not have any type, attribute, attack or defense stats.
So because of this, When Citadel checks attack values, checks all monsters the opponent has with attack values. Face-down don’t have any values, as stated, so it skips over those.
The “face-up monsters” isn’t stated on the card, because this is dictated by game mechanic.
If you have any further questions on this, please feel free reach out to the #rulings channel on the Duelist Unite Discord so that we can resolve this fully.
Any other bugs or issues you believe you find, feel free to open up another bug report so that we can investigate it!
Thank you for helping make Omega the best it can be!