I too was able to redeem first class awards with UA miles on the EK A380 from JFK but that was many months ago.
Now UA can't find availability for any gateway in first from the USA on any 777 flight not to mention YYZ on the A380.
Has EK zeroed out all Z class space from USA/Canada to DXB for an indefinite period?
Is there a way to check on the EK website?