There seems to be a general problem here, all of the above posts are very similar (which is reassuring, in a way ...). I too have updated my new iPhone 11 Pro Max to iOS 13.1 Beta 4, and done multiple soft-resets, reset Network Settings etc, but no difference.
I've always had my original full-sized Photos & Music on my phone, which is why I buy a 256GB handset. I have 23,000+ images in my most recent Apple Photos Library synced to iCloud, and the number still to be downloaded is coming down very slowly since Friday night from 8,000 to 6,000 etc.
Now that I know it's not just me I'll relax - I can imagine that iCloud servers are getting a hammering this weekend (millions of iPhone backups / restores) and they're melting, or maybe it's an iOS 13.x issue? Either way it'll get fixed & nothing appears to have been lost.
A good reminder that you always need (at least) a second physical backup of all of your photos and/or phone & not just a single cloud copy ...
EDIT: hmmm, "Downloading XX Originals" showing on my iPad now too - also on 13.x Beta - which I think are the new photos from the weekend, so probably not "a new iPhone" fault? My bet is iCloud constipation.