Well, as I said, I don't know much about it.
When I read the npc file, I noticed that only the "bandits" group was single-lined, the others were complete.
The only thing I did was put the same line that the "undeads" had more than the "bandits".
I did the server tests on new games, the same problem occurred before I added this new command line to the "bandits" group in the NPC file.
Enemy npcs did not take any damage.
After I added this line, it worked normally, both for new games and for existing games.
Other than that, I didn't find any abnormality after having done the process.
Maybe it's because the group doesn't have a related faction and we can't attack them.
After I put in the command line, it seems that this changed and I was able to attack normally like any other creature in the game.
I don't say this is a solution, but at least for my problem, I managed to "solve".
Below is the image of the only files I used.