I have Roon Arc on my iphone. Prior to the recent update I had downloaded music to my iphone but it now says these files are unavailable. I cant see how to download them again.
Seems like the latest ARC version removes your previously downloaded files……well it did for my downloads
I have them in a playlist so went back online and had to download them all again 
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Same happened to me, my Roon Arc on Android reset itself and wiped out all the previous downloaded files. I had to reconnect to my Core for the “First” time again and redownload the music for offline use.
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I have experienced this problem twice now. The ARC screen times out and then on reopening it the screen says “Synching with Roon Core for the first time”. All of the downloads are missing, though still there and taking up space, and re-downloaded them using data roaming (oops). Using Android and a Synology NAS core.
+1 Me to
Core for the “First” time again and redownload the music for offline use And it tacks so so long to download a Albums especially playlists SO SO long if it works at all that is… I’m still at it now…
Mac Mini M1 Core storage on Synology NAS
ARC on all iPhone 11 Pro, iPhone 11 Pro Max, iPhone 8, iPad Pro & iPad
Core for the “First” time
Different users, hence why it takes a very long time to get everything back up and running
Why does it take so long to download albums to iPhone Arc?
It did to me too. 90GB of them 
Also, on my iPhone albums won’t download if the phone blocks it’s screen. That means that I need to set the phone to never block when unloading and (even if I turn the brightness to minimum) the phone gets super hot to the point of slowing down to snail speed. I have had to put it into the freezer for big downloads (no joke, fact!)
As @Ricardo_Damborenea wrote, download stops if app is in the background or phone sleeps. It was said in other posts that this is generally an iPhone limitation for 3rd party apps.
As for speed and heat, there are hopefully improvements in the pipeline:
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