There's a similar surprising += 'problem' in JS. If you have the following code:

  async function totalSize(fol) {
    const files = await fol.getFiles();
    let totalSize = 0;
    await Promise.all(files.map(async file => {
      totalSize += await file.getSize();
    }));
    // totalSize is now way too small
    return totalSize;
  }

You get an overly low totalSize. It's caused by 'a += b' expanding to 'a = a + b', and the double-mention of 'a' creating a concurrency issue. If '+=' were a single operation with the right-hand-side being calculated first, it wouldn't be an issue.