Currently the Save and Continue does not save the Entry ID within the SQL wp_gf_draft_submissions table. There is an Entry ID within entry_meta, and this is created at about the same time. It would be great if the Save Submission also captured the new Entry ID if at all possible, OR if entry_meta also grabbed the gform_unique_id from the wp_gf_draft_submissions table.
For Save and Continue functionality, draft submissions are deleted upon form submission so storing the entry id has no benefit. The entry does not exist when the submission is just a draft, and when it’s a real entry, the draft submission is deleted.
The entry stored in the draft submission is a temporary entry only generated so it can be used when sending the form is saved and link requested notifications.