Incomplete external offset storage

[LeifW]

In your example for external offset storage, using plainSource, a single partition is assigned. I'm guessing it's common to subscribe to multiple partitions in one subscription? For those cases, I'm wondering about the edge case where some partitions had no offset saved before the system shut down. I think for those partitions, they should start wherever the "auto offset reset" setting dictates? Which is what plainPartitionedManualOffsetSource does.
Is there any way to subscribe to multiple partitions with plainSource, and only have a starting offset for some of them (i.e. skip the .seek call on the others?)
https://doc.akka.io/docs/alpakka-kafka/current/consumer.html#offset-storage-external-to-kafka