Losing the ball in the sun is a mistake by the fielder. Not a good job by the hitter. IMO right???
I wouldn't assume that losing the ball in the sun is a mistake by the fielder. It happens. What was the mistake? Failing to use the glove to shield? Failing to wear sun glasses? If that's the reason, then if falls under the category of a mental mistake, and mental mistakes are not fielding errors.
As far as whether the hitter did a good job, it's not the job of the scorer to judge that. The question is whether the fielder should have caught the ball at the moment it needed to be caught. At that moment in this case, the fielder couldn't see the ball, so it can't be fairly said that he/she should have caught it at that moment. Hard to catch what you can't see.
Also, someone said that perhaps scoring rules should be re-evaluated. Problem is, you've got a history of doing it one way, and to change that would then render MLB and NCAA records somewhat meaningless.