2 of the 3 visible swamps shown are on the smaller side, so it's not surprising that they don't have much in them. That southern area looks pretty big though... I've seen large swamps without crypts but it's unusual.

I'd say sail out a bit and find more swamps. If you think it might be bugged, plug your map seed into one of the map generator sites and see what it shows. Or if you don't want to spoil the exploration, share your seed here and somebody can verify or even give hints.