I really don't know how drop rates have changed, and I don't hava a sample size big enough to really tell me anything, so your droprate could just be bad luck, or working as intended.

Since I'm pretty casual with this game, I don't really hunt down every event girl, I skipped the ones in the last short event to get more money, I really wanted to get some 5 stars done, great to know the next ones are around the corner and I can't even 3 star the last girl that dropped while farming money.

Anyway sample size, 5 regular girls dropped by bosses since the changes, so that's around the same as before.