Looking back
The player chooses to interact with the chest. The player is told what happened and given details about the thing they are making a save for making it too late to retcon in butbutbuts. D&d is not like fate or something where you can spend some kind of resource to
declare things into existence. "I would have stopped had I known there was a truck" doesn't change things while screeching towards a car crash because it's too late. Even with fate's declarations players & GM can't simply
nosell something completely after it's presence has been established. D&D
once had a subsystem that would have allowed this sort of retcon to function as something other than straight up nosell the poison that got "sufficient telegraphing" but that subsystem is not in 5e.