Daylight saving shifts job starting times?

Yes you did. Apologies.

If you pick it up, you can consider (probably after code study) whether or not going “full cron” is best, e.g.

If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately.

sounds to me (and you can confirm – you have more cron skill) that you get a burst of jobs in the Spring.

1:45 job every 30 minutes
1:45 AM
2:15 AM doesn't happen
3:00 AM roughly 15 minutes from previous
3:15 AM roughly 15 minutes from previous
3:45 AM

I’m proposing that the chosen interval be maintained, basically just pick the right UTC for the next time.

1:30 job every 30 minutes
1:30 AM
3:00 AM
3:30 AM

Duplicati does have a missed-job practice of ASAP run when Duplicati starts. Maybe it could be used,
but I kind of think staying on requested interval is better. I don’t administer systems. Feedback sought.

Of course almost anything other than the current plan is an improvement, so where it lands may vary.

Scheduling strategy for backups has design information from the original author to credit for Duplicati.
Everybody wishes it were better than it is (ever getting out of Beta?), but the resources are super thin.

I didn’t start the project, I don’t run it (but I do have a little bit of say), and I wouldn’t be writing the code.