Graveyard - Tested on Acolyte mob, cause he isn't Undead. The mob can't be added to a group, adding creeper to the same group works fine and extra damage and effect are applied to Creeper but not to Acolyte. Tested both on Fabric and Forge+Sinytra Connector.
Realm RPG: Imps & Demons - (No fabric version) Can't add any mob. They have a "demon" tag tho, which I tried to change to "outerworldly" for example but it didn't fix anything.
Realm RPG: Creep & Crop - (No fabric version) Same
I tried adding mobs to different groups with the same result.
Sorry if it's not a bug but me doing something wrong.
{
"replace": false,
"values": [
"minecraft:creeper",
"graveyard:acolyte"
]
}