That's interesting. Do you have snails? Maybe some snails took advantage of the melting crypts and finished them off? I don't know what to tell you, other than I've had crypts come back from the grave after 6 months of thinking I removed every last trace of them! It's usually not a good thing, either, because when they decide to resurrect from the dead, they always try growing up through a thicket of rotala or didiplis.
Speaking of didiplis, I also once experienced a spontaneous regrowth of this plant after thinking I had eradicated it. Unlike the crypts, though, this one came back in the perfect location, and the look of it was stunning, because it came up as individual stems, nicely spaced apart from each other, as opposed to the big bunch of them I had originally planted.
I still love crypts, though. How could I not? My 5g is filled with them, even though I never fertilize, I rarely change the water, and seldom add Excel.
They are the low-light plants, for me.