The provider is really irrelevant. It's a plan asset, and a plan problem. How much of an issue depends on the details. When was the original payment made, was there notice and consent or was it a forceout, were there taxs withheld, was it reported on a 5500, 1099, 945, 8955, etc...
It should be deposited to the trust, he participant should be default enrolled, and payment process should start over (and other steps as necessary). If participant does not receive your payment, go to your missing participant procedures.
I do not think that sending the check to the sponsor is reasonable as it should be handled on a plan level.