Exporting "Spouse of" or other relationship with matching labels only exports the A>B relationship
When exporting a contact and their related contacts, the relationship type dropdown truncates all the "duplicate" relationship labels. So when you a user selects "Spouse Of" it only exports the A>B relationship.
If you were to export John Doe(A) and James Doe(B), who are spouses, the export would be rendered as
|First||Last||Spouse of First||Spouse of Last|
This is an issue if you are trying to export a list of donors and spouses. If the primary donor is the B spouse you will end up with no spouse exported.
This appears to be an old issue. https://civicrm.stackexchange.com/questions/27001/exporting-spouses-only-exports-one-side-of-relationship