At the tail end of truncate, is this logic:

// If we've been given a size greater than we are
if (newSize > currentSize) {
    // Reset the position only
    this.position = newSize;
}

which is not what the API says: "If the given size is less than the current size then the entity is truncated, discarding any bytes beyond the new end. If the given size is greater than or equal to the current size then the entity is not modified."

I think you wanted this instead, which should be inside the above array truncation:

// If we've been given a size less than we are
if (newSize < currentSize) {
    // Reset the size only
    this.size = newSize;
}