@Okgo Please tell us what film you used. Given that you mentioned the 20% reduction, I'm guessing it's likely Llumar's AIR 80. Is it? I'm also about to add that to my windshield.
As mentioned above, the temperature of the glass surface is likely to heat up because all these ceramic tints, including Spectra Phototsync, are absorbing the energy. They are not reflecting it. Note also that the film goes on the inside of the windshield, and not the exterior, so again it makes sense (to me) that the surface would be hotter.
Having said that, the net reduction within the vehicle should still be less, but I'm not sure how to prove that conclusively. With the film, the AC should work less & use less energy to cool down the interior of the car than it would without it. Without the film, there are plenty of posts which show the temperature of the seats & dash itself are 140 degrees F so in total you probably have a lot less mass within the car that has absorbed that solar energy and would be radiating it back at you. Can I prove it? No, but I am confident that the high temperature on the glass is not indicative of the overall benefit.
I am still likely to add the AIR 80 to my windshield, but I'm very interested in your situation.. I'm curious if the overall car interior is cooler, but there is more noticeable heat immediately close to the glass. That is a downside that I hadn't considered until seeing your post, and the situation seems credible. Given the reduction in UV & IR, I would hope that any air movement within the vehicle would still net a cooler feeling to the top of my head which lacks any blocking or insulating layer of hair.
Please tell us if you used Llumar Air 80 or something else. Please also tell me if you see any visual distortion, particularly at night. I've read various posts indicating that these films were not intended to be viewed at an angle, but more straight-on like they are on side windows.
