Structure voids aren't part of the "#minecraft:dragon_immune" or "#minecraft:wither_immune" tags

Can confirm in 21w06a.

Can confirm in 21w07a.

Can confirm in 21w11a.

Can confirm in 21w13a.

Can confirm in 21w15a.

Can confirm in 1.17.1.

I can confirm this behavior in 1.18 Release Candidate 3.

Here's a potential fix regarding this issue. The following is based on a decompiled version of Minecraft 1.17.1 using MCP-Reborn.

Potential Fix:

In the net.minecraft.data.tags.BlockTagsProvider.java class, simply adding Blocks.STRUCTURE_VOID within the DRAGON_IMMUNE and WITHER_IMMUNE block tags should resolve this problem. The correct lines of code within this particular class should look something like the following:

net.minecraft.data.tags.BlockTagsProvider.java

public class BlockTagsProvider extends TagsProvider<Block> {
   public BlockTagsProvider(DataGenerator p_126511_) { super(p_126511_, Registry.BLOCK); }

   protected void addTags() {
      ...
      this.tag(BlockTags.DRAGON_IMMUNE).add(Blocks.BARRIER, Blocks.BEDROCK, Blocks.END_PORTAL, Blocks.END_PORTAL_FRAME, Blocks.END_GATEWAY, Blocks.COMMAND_BLOCK, Blocks.REPEATING_COMMAND_BLOCK, Blocks.CHAIN_COMMAND_BLOCK, Blocks.STRUCTURE_BLOCK, Blocks.JIGSAW, Blocks.MOVING_PISTON, Blocks.OBSIDIAN, Blocks.CRYING_OBSIDIAN, Blocks.END_STONE, Blocks.IRON_BARS, Blocks.RESPAWN_ANCHOR, Blocks.STRUCTURE_VOID);
      this.tag(BlockTags.WITHER_IMMUNE).add(Blocks.BARRIER, Blocks.BEDROCK, Blocks.END_PORTAL, Blocks.END_PORTAL_FRAME, Blocks.END_GATEWAY, Blocks.COMMAND_BLOCK, Blocks.REPEATING_COMMAND_BLOCK, Blocks.CHAIN_COMMAND_BLOCK, Blocks.STRUCTURE_BLOCK, Blocks.JIGSAW, Blocks.MOVING_PISTON, Blocks.STRUCTURE_VOID);
      ...