If you leave any of the fields for ApplicationUser OR IdentityUser null the update will come back as successful but wont save the data in the database.
Example solution:
ApplicationUser model = UserManager.FindById(User.Identity.GetUserId())
Add the newly updated fields:
model.Email = AppUserViewModel.Email;
model.FName = AppUserViewModel.FName;
model.LName = AppUserViewModel.LName;
model.DOB = AppUserViewModel.DOB;
model.Gender = AppUserViewModel.Gender;
Call UpdateAsync
IdentityResult result = await UserManager.UpdateAsync(model);
I have tested this and it works.