Sorry for the late reply - life.
Thanks for the suggestions. I had the same firmware update so thought I’d just wait and see if that fixes things - apparently not as I had the same problem today. My current thought process is that when I update the roon core via the in-built update process (Win32 client in my case) and it obviously updates and restarts the QNAP core, perhaps after that we start to see problems. I had it running fine for ages until the latest update that I applied a day or so ago (for the airplay2 compatibility). Then 2 days later - 32gigs gone.
Just an idea. Perhaps I need to manually restart roon via the QNAP interface post update.