Why is ClaimTypes.NameIdentifier not mapping to ‘sub’?

1. To not let Microsoft Identity to override claim names you have to use JwtSecurityTokenHandler.DefaultInboundClaimTypeMap.Clear(); just before the app.UseAuthentication() in the API startup.

2. Use direct “sub” claim instead of ClaimThypes.NameIdentifier e.g.
   var id = this.User.FindFirstValue("sub");

For further reference please see detailed discussion on it:
https://github.com/IdentityServer/IdentityServer4/issues/2968#issuecomment-510996164