People said that it's a stupid move from anastacius but I think it's a gamble. Cause out of everyone there, he's the only person (beside protagonist side ofc) who have the right to check on "my little brother" imo. Maybe right there & then, he wants to reveal Claude's use of black magic & strip him off of the throne while all nobles watching. This makes me "imagine" a lot of different scenarios of what will happen next ...
If Lucas is still in the palace (waiting for Athy to finish her meeting maybe), there's no way Lucas didn't detect that presence of black magic coming from Anastacius