There are several reduced solo supplements in March, April , May but they are not advertised and you have to go through the booking process to see them.
two that I have seen are the Greece, Turkey and Bulgaria on May 2 and the Wine cruise on May 30. There are several in March too including Spain, France and Italy.
I have no idea why they are not listed on the site when you search solo cruise deals but I found them by chance when I did dummy bookings.
I don’t know about June yet, there may be some there too. Many are also listed on Quarterly savings for an extra 10%.
Anne