What mechanism are you using to add a NEW (manual) individual approval ?

I am asking because most likely you created this feature (customized) and I don't have it on an out of the box instance.

Anyway, I would suggest you to go into the mechanism you created (UI action, etc.) to ensure that you are pushing a value into the document_id field, copying it from the sysapproval field.

Maybe something like current.document_id=current.sysapproval;