We owned a property with tenants and have since sold the property. We believed both tenants to be month to month, but one has since produced a lease. Who is responsible for honoring the lease? The new owner wants the property vacated

I would say that the new owner has to honor the lease. He knew that it was occupied when he purchased it. Did your sign the lease that was produced?

That is what is being alleged, if so, now what?

Most lease agreements would have a clause that says something about - in the event the property changes ownership.
I would first ask to see a copy of the lease.
What or who is the problem. Is the new owner coming back to you? Or are the tenants?

Both, the new owner wants the tenant out, the tenant refuses to leave
I have another lease agreement with the same wording for another tenant, there is nothing about selling the property

Although it may not reference selling the property in the lease, you should check out the Landlort Tenant Act in your state.

I am sorry, you owned property and did not know if there was or was not a lease on the property,? Sorry you would be the one doing the lease.
If the lease is expired, and old, it has no value.

If there was a leave, and you did not notify the buyer that they were buying property with a lease, you may be liable to the buyer for non disclosure.
but if the tenant has a valid lease, the buyer has to honor it. So the lease is in place and the buyer can not void it.

Now the buyer can possibly buy the lease out, but you may find that they could sue you for that buy out, if you did not disclose the lease and their intent was to take possession of the property at closing.

It standard real esate law that a lease has to be honored by the buyer, and has to be disclosed in the selling agreement.

The tennatns have the rights and can not be make to move, the buyer may come after selling for damages.