Curiously enough, so far I've only ever observed the reverse happening - spammers somehow ending up stripping off the common prefix and leaving the individual suffix intact.
(Though on the other hand thankfully I'm still not getting much spam anyway and it's only a few addresses that were affected, so I only have a limited amount of data to go by.)
Not really surprising: in the eyes of a naive regex for extracting email addresses from a string, a+b@c.d contains a nicely email-looking substring b@c.d but a@c.d isn't contained at all. Spammer address supply chains aren't usually characterized by the highest levels of sophistication.
(Though on the other hand thankfully I'm still not getting much spam anyway and it's only a few addresses that were affected, so I only have a limited amount of data to go by.)