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For some reason the brightness of my screen dims at times. Power saving mode is off and I have 80+% battery.

It was not checked. and I also had the other setting on dynamic
Any chance your thumb (or other finger) is covering the ambient light sensor? It's one of the little openings at the top of the phone, by the ear piece. If you're inadvertently covering that, with the screen brightness set to dynamic, it'll faithfully adjust to the perceived ambient brightness.
The screen will dim when the phone starts to get too hot! When its on full brightness it will heat up quite quickly and the thermal circuit on the CPU will detect that the CPU and main board is getting too hot and will dim the screen to try help cool the phone! Try having the phone set to 80% brightness and it should stay there a bit longer. Try to keep your phone cool too!