Hmm.... If you create new NPCs, that's really strange. Did you rename your plugin file? This would explain the issue.
At any rate, check your Facegen folders. These are:
.../Skyrim SE/data/meshes/actors/character/facegendata/Facegeom for the head meshes and
.../Skyrim Se/data/textures/actors/characters/facegendata/Facetint for textures, especially tintmasks.
BTW, folder names in Skyrim are NOt case-sensitive. It doesn't matter if the folder is e.g. "FaceTint" or "facetint".
Within these folders, there MUST be a subfolder named after your mod's .esp. This folder must have the exact same name as your plugin. E.g. if your mod's plugin was called "MyNewNPCS.esp", you need a folder named "MyNewNPCS.esp" both in the "facegeom" and "facetint" folders. Check this first. You also must not rename any files within these folders.
My guess is that the game can't find (or read) the tintmasks. Maybe re-exporting facegen data can solve the problem.
If you changed the name of your plugin, you have to manually change the name of the subfolders in .../facegendata mentioned above to the new name of your plugin.
If you're working from Racemenu presets, make sure to copy tintmaks you created in Racemenu to .../facegendata/tintmasks/[name of your mod] and give them the exact same name as the tintmasks created with the CK.