really? Which poskim?
I think you are misunderstanding something you heard.
I'm at work and cant tell you right now who holds till Shavuos, but this is from the Star-K website who brings from Rabbi Heinemann about waiting 4 weeks post pesach:
"However, if a person has the option of shopping at a store which does not receive goods from a Jewish distributor that may have owned chometz on Pesach, it is commendable to do so until four weeks after Pesach.6"
"6. Heard from Rabbi Heinemann shlit”a"
When I get home I wil bl"n find you the Poskim that hold till shavuos. It may be that these days chametz replenishes much faster. And again, this is a minority shita and most don't hold by it.