Someone (even just recently!) posted a good explanation of some of the programmatical challenges of making such a change: DST changes scheduled backup time · Issue #1364 · duplicati/duplicati · GitHub
While I haven’t looked at any of the code myself, this summary is also assuming that Duplicati has no built in assumptions that the timestamps of all backups are monotonically increasing. For instance, does it even compare the timestamps of two versions to see which is newer or older? What if two have the same exact time? (<=hourly backups when crossing the Fall time change). Again, not saying this is what Duplicati is doing but as mentioned earlier, it depends on how deeply ingrained these timestamps are.
On the plus side, as someone mentions in that github issue thread, countries continue to solve this problem for Duplicati by getting rid of DST. I can’t wait until they do in the US.