Lumia 950XL display became very dim (not related to brightness setting)

My phone display will randomly start dimming (very dim such that everything is barely readable). I tried to lock/unlock, or change the brightness setting (I disabled auto brightness) and changing the brightness to high, but that didn't do anything. The only thing is to reboot the phone. Lately this has been happening daily. Any idea what is causing this? Hardware failure or software?

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