Thank you for your post.
At what point does the data go missing? That is, after you enter the data in the portal row and commit the record, does the data disappear immediately? Or, at some point later when you revisit the record do you notice the data missing?
Is the portal part of a Tab Control? Is there an image behind the portal? If you create a new layout with just the portal, does the information then display?
Does the relationship allow deletion of related records? If that is the case, when you delete a record, the related records would also be deleted.
Any other information you can provide may be helpful in narrowing down the possible causes.