iTunes Library deleted after auto-renewal of iTunes Match

My iTunes Match subscription automatically renewed a couple of weeks ago, at which point, the majority of my music library disappeared from my iTunes Media directory. A small handful of files were untouched; the only common element of those is that those directories were newly created in the past year and contained music unlikely to be matched (local indie music mostly). Strangely enough, my iTunes had iCloud Music Library unchecked, so I was surprised that anything happened.

Going back to Time Machine backups shows that the files disappeared about a day after the renewal. While I go through the process of restoring the backups, are there pointers yet to avoid this phenomenon happening again?

To complicate matters a little, my music library is stored on a PX6-300D NAS for (apparently not enough) data protection. This did initially give iTunes Match a bit of heartburn, but it's been running fine for months.

Thanks for any suggestions.