An answer to the question How detectors in particle accelerators can differentiate neutrons from antineutrons do not show that an antineutron measurement was successful. The answers are from 2014 and I’m curious about the current situation around the measurements of antineutrons.
Single interactions with antineutrons have been seen in bubble chambers.
The original paper in this link.
Please note that such events prove experimentally the existence of antineutrons. The difficulty with high energy experiments and detector lies in the high energy showers created by all particles. One has to rely on tracking detectors to define charged particles, and the antineutron is not charged, then on calorimeters to get momentum and energy , but it is not possible in the calorimeters to identify the particle if it does not connect with a charged particle in the particle detector.
It is hard enough to make a neutron beam, as they cannot be controlled with electric and magnetic fields. Neutron antineutron oscillations are another story, which has not materialized experimentally, but has nothing to do with the existence or not of the antineutron.