The
NY State Dept of Health says
"You may also dispose of the cremains in any manner that complies with local health department or other regulations."
...so legally, it's a local issue.
I couldn't find an answer addressing whether an owner or agent is required to disclose, but my R/E knowledge leads me to think "No"
Even if someone "broke the law" in burying them there, I don't see that a case could be made that it's knowledge would affect the transaction; That is, be a detriment that a new owner needs to be aware of.
If you don't get a firmer answer, try the NY City Dept of Consumer Affairs. They've got some info
here, but don't say what is acceptable for ashes.