I get that, too, but not very often.  I wonder if it has something to do with the automatic brightness adjustment that happens with changes of lighting in the environment?  I guess there's only one way to find out and that would be for me to turn the setting off and see if it stops.

It doesn't really annoy me too much, though, and I can live with it as long as there is nothing about it that signifies something will soon malfunction.

Use lux autobrightness from play store. That should eliminate the issue