Regarding resuming backups: even though duplicacy is better than duplicati, we have to be honest that that’s not so difficult to achieve since, as you point out, duplicati doesn’t resume at all. But there is room for improvement with how duplicacy resumes too:
Ok I see, still some work to do
Did I got it right that only windows is affected?
I was testing on a Linux based system and never had such an issue.
I’m using Windows, but I don’t see why duplicacy would operate any differently on another OS, so I’d say, no, this problem is not specific to Windows.