Unsure if this happened before the .30 update but on the latest release the wither isn't applying the wither effect on worlds set to hard difficulty.

Steps to reproduce:

1. Set up a server and set it to hard
2. Summon a wither
3. Let it hit you with a black skull. 

Expected result: Player takes damage and gets the wither effect

Observed Result: damage is taken, but no effect.

This appears to be fine on normal worlds. linked videos for normal worlds and servers (I dont have realm to test but someone else did). 

As a further note, none of these tests have any behavior packs enabled either.

3 Items