If a merchant loses a check, is the customer responsible to reissue
another one when the merchant requests it? Or is it the customer's good
luck?

Assume that the customer and merchant have some bad blood; maybe the
merchant refused to accept something the customer wanted to return, and
that this lost check turns out to be an opportunity for the customer to
get some just desserts. Can the customer say "ha! I gave you a valid
check, my responsbility ended there. You lost it, you eat it."

--riverman

(of course, this is hypothetical, and anything that is said here is
merely hypothetical and not really legal advice...)