Seen this before. You might be able to reproduce it like this
A and B are duplicates. They are both married to Y, but with conflicting details.
A is merged into B, but the marriage records aren't merged, because of the conflicting details.
Now, A has gone away. B still has the marriage to Y, with the same details as before.
Y now has 2 marriages, to A and B, with different details. But if you click on the link to A, you get redirected to B.
You can now sort out the details, put the correct info on the Y-B marriage and remove the Y-A marriage (at the Y end).
(You can also exploit this feature if the same couple marry twice, though you only see both marriages at one end)
But if you remove the Y-B marriage, you're left with a Y-A marriage that redirects to B, but no way back from B to Y.
Dunno if conflicting marriage details are still handled the same way in the new system.