Interesting. Not so sure this is an Apple problem. Seems just as likely to be a problem with your NAS. If I started getting that I might be inclined to start fresh on that NAS with an empty Time Machine volume and see if the problem goes away. At any rate, always good to have multiple Time Machine devices anyway as additional protection. I also use a USB drive now and then.It would do a complete backup every time (and not just the incremental changes). It filled a 2TB drive within a day or two. It started happening about a year ago. I switched to a drive directly plugged into a Thunderbolt hub instead.