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one things to note first... It might be that the device's limitation isn't just enough for you (honestly it isn't enough for me in some cases), meaning the brightness of the phone is maxing out but just isn't strong enough to overcome the sunlight.
Other than that, the device might really be below standard in which case you just happened to have drawn the short straw OR if you had the phone for a while the phone is just degrading a bit faster than it should. In either case the hardware relate issues are often very random even if the quality controlling is done very tightly.
In such case you could take the phone to UbreakIfix and have it checked whether it's working as it should or underperforming. (my friend uses latest iPhone model and still noticed the screen wasn't bright enough and had hard time controlling his drone as well, so it's not necessarily Samsung's fault).
Another possible issue is if the brightness is being lowered to like 10% or 20% or anything below 80% in auto adjustment enabled... it's a software issue that needs to be reported to Samsung Members app.