Expected behavior:

changing:

"minecraft:attack": {
  "damage": 4,
  "effect_name": "wither",
  "effect_duration": 10
},

to:

"minecraft:attack": {
  "damage": 4
},

should remove the wither effect

Actual behavior:

In 1.9.0 (and possibly earlier versions) it does not.

How to reproduce:

With an existing behavior pack, take the wither_skeleton.json from vanilla.zip or

Vanilla_Behavior_Pack_1.9.0.zip and edit it the attack component as above

Problem reproduced with "format_version": "1.8.0" and "format_version": "1.1.0"

Notes:

This was not a problem for whatever version we were at for the release of my marketplace map in july 2018.

I'm not sure exactly what version introduced this problem, but I suddenly noticed it with 1.9.0 with my existing map on the marketplace.

At the moment I'd probably have to avoid using the entity entirely if I want it to attack without the wither effect.