Program: 11671
Project: 11671
Buggy source:
/** 
 * Updates the usability state field in order to check if this chunk may still be used or is reclaimable.
 */
public void checkValidForGc(){
  this.useState.set(TRANSITION);
  if (this.holders.isEmpty()) {
    this.useState.set(UNUSABLE);
    if (this.world.removeChunkAt(this.x,this.z) != null) {
      Region region=Region.getFile(this,true);
      int rX=this.x & 31;
      int rZ=this.z & 31;
      if (region.hasChunk(rX,rZ)) {
        try (DataInputStream in=region.getChunkDataInputStream(rX,rZ)){
          Tag.Compound root=Tag.decode(in);
          Tag.Compound compound=root.getCompound("String_Node_Str");
          CompletableFuture.runAsync(() -> this.write(compound),ARBITRARY_POOL).whenCompleteAsync((v,t) -> {
            try (DataOutputStream out=region.getChunkDataOutputStream(rX,rZ)){
              root.write(out);
            }
 catch (            IOException e) {
              throw new RuntimeException(e);
            }
          }
,ARBITRARY_POOL);
        }
 catch (        IOException e) {
          throw new RuntimeException(e);
        }
      }
 else {
        ARBITRARY_POOL.execute(() -> {
          Tag.Compound root=new Tag.Compound("String_Node_Str");
          Tag.Compound level=new Tag.Compound("String_Node_Str");
          root.putCompound(level);
          this.write(level);
          try (DataOutputStream out=region.getChunkDataOutputStream(rX,rZ)){
            root.write(out);
          }
 catch (          IOException e) {
            throw new RuntimeException(e);
          }
        }
);
      }
    }
  }
 else {
    this.useState.set(USABLE);
  }
}

Fixed version (ground truth):
/** 
 * Updates the usability state field in order to check if this chunk may still be used or is reclaimable.
 */
public void checkValidForGc(){
  this.useState.set(TRANSITION);
  int centerX=this.world.getWorldOptions().getSpawn().getIntX() >> 4;
  int centerZ=this.world.getWorldOptions().getSpawn().getIntZ() >> 4;
  if (this.holders.isEmpty() && (Math.abs(centerX - this.x) > 3 || Math.abs(centerZ - this.z) > 3)) {
    this.useState.set(UNUSABLE);
    if (this.world.removeChunkAt(this.x,this.z) != null) {
      Region region=Region.getFile(this,true);
      int rX=this.x & 31;
      int rZ=this.z & 31;
      if (region.hasChunk(rX,rZ)) {
        try (DataInputStream in=region.getChunkDataInputStream(rX,rZ)){
          Tag.Compound root=Tag.decode(in);
          Tag.Compound compound=root.getCompound("String_Node_Str");
          CompletableFuture.runAsync(() -> this.write(compound),ARBITRARY_POOL).whenCompleteAsync((v,t) -> {
            try (DataOutputStream out=region.getChunkDataOutputStream(rX,rZ)){
              root.write(out);
            }
 catch (            IOException e) {
              throw new RuntimeException(e);
            }
          }
,ARBITRARY_POOL);
        }
 catch (        IOException e) {
          throw new RuntimeException(e);
        }
      }
 else {
        ARBITRARY_POOL.execute(() -> {
          Tag.Compound root=new Tag.Compound("String_Node_Str");
          Tag.Compound level=new Tag.Compound("String_Node_Str");
          root.putCompound(level);
          this.write(level);
          try (DataOutputStream out=region.getChunkDataOutputStream(rX,rZ)){
            root.write(out);
          }
 catch (          IOException e) {
            throw new RuntimeException(e);
          }
        }
);
      }
    }
  }
 else {
    this.useState.set(USABLE);
  }
}