NEB fixes the first and last images?

Dear LAMMPS users,

Recently I used the NEB function in LAMMPS to calculate the transition path of Silicon atom on amorphous Si surface. All the first/last images were fully optimized using CG method, and it can be verified that they were local stable configurations. For most cases, NEB can provide reasonable results, however, I found some strange cases. After NEB calculations, the first/last images move to a nearby stabler configuration. So my question is, if the NEB implementation in LAMMPS fixes the first and last images? In principle, they should be fixed during NEB and only the intermediate images can be relaxed. I dug into the source code and checked min_fire.cpp, neb.cpp, and fix_neb.cpp, and none of them shows such constraint. Before modifying the source code, can I have any way to realize the fix in NEB?
Furthermore, I found that for some cases NEB completely failed. The total potential energy becomes a very large value which means that the system is melted after adding only one adatom on the surface. Do I have better way to calculate the transition path in LAMMPS?