This is currently a known limitation in ARC: If you switch to a new server or even just restore a backup, it is considered a new server and ARC must re-sync. One of the consequences is that it loses the downloads.
For backups, a solution is hopefully on the horizon. I’m not sure if this extends to new servers:
This quote was from 13 days ago and there was a regular release since then, but I can’t find this item in its release notes, so maybe it is still on the way.