We found a bug on the antineutron annihilation at rest. If you generate an antineutron with some kinetic energy, it will annihilate with the target atom and produce some secondary particles/annihilation products. But if it is at rest (0 kinetic energy), it doesn't do any interaction at all. 

This is also seen in the antideuteron annihilation (kinetic energy ~!keV) with some target atoms. It seems that GEANT4 treats an antiproton and an antineutron separately in the antideuteron annihilation; one of the antiparticles (particle-1) first annihilates with the target atom, but the other (particle-2) stays at rest. If particle-2 is an antiproton, it will then do annihilation, but if it is an antineutron, it won't do any interaction as discussed above. 

Also, some specific X-rays will be produced in the annihilation process (FTF model), but they were not seen in the old GEANT4 (CHIPS model). Could you let me know what kind of physics process produces these X-rays? 
(I submitted a question in the GEANT4 hypernews before: http://hypernews.slac.stanford.edu:5090/HyperNews/geant4/get/hadronprocess/1464.html)

Thank you for your time and help.