I thought that was the way it works. Are you saying you see the alt URL error even when things are fine?
Download links not working anymore shows my CLI check used to error. It’s fine since update server fix.
Now if you meant can Duplicati better handle the primary server not working, I could agree with the wish. The messaging could be better, but ideally the alt server would exist, not have the same issue, and work.
Squashing the DNS error might be as easy as having alt DNS-resolve to same IP as primary, i.e. a retry. That would leave the installed Duplicati code the ability to go elsewhere via DNS if server admin set that. But the elsewhere would still have to exist, and preferably not updated on the same schedule as primary.