Releasing black warlocks doesn't grant you any bonuses. (It is said that releasing black warlock will increase Strength level by 15% plus 5.)
I also tried releasing snowy knights, but still no bonus.
I can't confirm this, but I'm pretty sure that the same thing occurs with releasing any kind of butterflies.

I tried restarting the client, but even then no bonus was granted.

I'm not sure if it's necessary, but I'll add a screenshot with the jar's ID.