FE NXT Batch import to an existing batch

We're trying to import JEs into an existing batch that has JEs in FE NXT webview using import and getting duplicate errors, even though these aren't duplicate entries.

We've tried using just the batch number with that as the match for the import. As well as Batch Import ID and adding a batch ID, such as 100-50, where 100 is the batch number and 50 is the next empty row and we similarly get duplicate errors.

We are able to import (with that same file) successfully to a new batch. Though this means we'd be posting a lot more batches and this is less efficient. We prefer to be able to add JEs to an existing batch and post at one time.

I can't find any recent documentation in knowledgebase or forum on this. Has anyone been able to complete this import to an existing open batch?

Thanks,

Richard.