Here's my slightly educated guess (I worked at Apple and for a time iTunes adjacent) the empty space is for file watermarking on edge servers. All iTunes (Appl

Here's my slightly educated guess (I worked at Apple and for a time iTunes adjacent) the empty space is for file watermarking on edge servers. All iTunes (Apple Music) content is served via CDN. By leaving some empty space in the file the edge server can just write the owner's account ID fingerprint without having to remux the whole file. Apple can just send a virgin copy of a song to the edge servers and they can do the customization inline with serving it. That will be computationally cheaper than remuxing it and require no extra storage since the file is tweaked in flight. Leaving a lot of space provides enough room for some worst case size of watermark data without producing invalid files. But that's only my guess. I haven't looked at any files in question.