The “partner’s partner shows when viewing G603 record in the family navigator pane.
Actually it shows “partner’s partner” for both/either wife in relation to the other. It also remains the same whether you are logged in or not. And, as you said, 1 MARR Y exists on both family records.
Furthermore – I can see the same thing on my own site (in my own personal record!). So we have a bug in the relationship code. I will move this to the bug reports and fix it as soon as I can find where the error is.