The bug

Structures can provide multiple palette variants, however their selection looks faulty:
(MCP: net.minecraft.world.gen.feature.template.PlacementSettings.func_204764_a(List<List<BlockInfo>>, BlockPos))

public List<cci.b> a(List<List<cci.b>> list, @Nullable et et2) {
    this.j = 8;
    if (this.j != null && this.j >= 0 && this.j < list.size()) {
        return list.get(this.j);
    }
    this.j = this.b(et2).nextInt(list.size());
    return list.get(this.j);
}

There are two checks (non-null and >= 0) which will always succeed. And for structures with more than 8 palette variants (currently not the case for vanilla structures) always the 9th palette variant is loaded.

How to reproduce

Seems like variant selection is position dependent, therefore the following steps load the structure at multiple positions.

Structures have to be placed inside the generated/minecraft/structures directory of a world.

1. Load the structure 8-variants.nbt at multiple positions

   /fill ~1 ~1 ~1 ~10 ~1 ~10 structure_block[mode=load]{mode:"LOAD",posX:0,posY:1,posZ:0,sizeX:1,sizeY:1,sizeZ:1,name:"8-variants"}
   

   /fill ~1 ~ ~1 ~10 ~ ~10 redstone_block
   

   → Different wool types are placed

2. Load the structure 9-variants.nbt at multiple positions

   /fill ~1 ~1 ~1 ~10 ~1 ~10 structure_block[mode=load]{mode:"LOAD",posX:0,posY:1,posZ:0,sizeX:1,sizeY:1,sizeZ:1,name:"9-variants"}
   

   /fill ~1 ~ ~1 ~10 ~ ~10 redstone_block
   

   → It always places the 9th variant, light gray wool

How to verify code-wise

1. Decompile Minecraft or use a viewer such as Bytecode Viewer
2. Search for the string

   fossil/spine_1
   

3. In the source look for the first line of code containing

   16 - 
   

4. Open the type of the local variable two lines above
5. Search for a method containing

    = 8;