I think that the underlying technical issue is that under long-standing case law, a taxpayer cannot "turn his/her back" on income, from any source, without being in "constructive receipt" of the income (i.e., it's included in his/her gross income). Receiving the check and not cashing it is "turning your back on income" if you understood that what you had received in mail was a check.
Although the plan administrator would, almost certainly, have no way of knowing, arguably you would get a different result if the taxpayer had moved and did not get the check, or if he or she had thrown it away without opening the envelope, not realizing a check was inside (which might be impossible to do, depending on the envelope).