I think they do. Especially Zoom. They appear to put off an oily substance after a year or so.
I have gotten to the point that I don't buy any in bulk, especially Zoom.
I don't mind buying a hundred pack of Berkley power worms, they seem to hold up for a full season without degrading at all.
Not sure about some of the 'custom' stuff. Don't know if those guys have all the additives to make the worms last forever. Course since most of them don't sell in quantity higher than 10/pk it's not a big deal.