Exporting models and animations from the game for use in Unreal Engine.

I've noticed similar defects. Perhaps it will help if, after loading the SMD file, you select all the vertices and press Break, then select the vertices again and press Weld Vertices with a value of 0.001 (demonstrated in the video).
I also noticed black normals on the models. You can fix them by unchecking "Rebuild smoothing normals" when loading the SMD file and then manually assign smoothing to the polygons (demonstrated in the video).
The SMD plugin from Wonderboy causes some defects, so I used the plugin from NeilJed.

Yes, I tried doing all the actions with the vertices as shown in the video, and I unchecked "Rebuild smoothing normals," and then did Autosmooth as shown in the video, but unfortunately, it didn't help. I can't figure out the reason, as I am a programmer myself, but I'm a complete beginner in 3D.
Regarding the SMD plugin from Neil 'Jed' Jedrzejewski, I later noticed that it's the same as Wonderboy. I've attached a screenshot from the import screen, where you can see his name and his nickname.

There's also this SMD plugin from GameZombie for 3ds Max versions 2014-2022 https://knockout.chat/thread/806/1
It seems to import the mesh with the skeleton without problems, and I was able to apply FBX animation without defects, but unfortunately, this plugin doesn't support SMD animations. Therefore, the most optimal option seems to be Blender with the SMD plugin; so far, I haven't noticed any problems with it.