Indeed, Anastasia's question about why in the world Lyssandre suddenly changed his mind had been answered by someone she truly didn't expect.
It's not like him to take back his words, after all. Just like when he finally decided to support his son's dreams despite relentlessly opposing them since forever - that only happened for a reason, which turned out to be betrothing him to a complete stranger.
Now, the cause behind Lyssandre approving Lucien's request to hold a party in their mansion had ended up to be Eclair again.
Anastasia shouldn't have been so thunderstruck that she found it preposterous, though. After all, she already noticed once before that he was more generous to her than to his own son.