Saved Albums keep on un-downloading themselves!

[Dugu]

I've been a premium (family pack) user for years. Frustratingly, my downloaded albums, of which I have, currently 2,900 tracks, occasionally just all get un-downloaded. I can't figure out if there's a 'download all' setting as it takes me close to 20 minutes, every time, to select each album to download again - and it obviously takes an age (hours) to download all these tracks again. I've got a OnePlusOne.