I have this issue since the last update as well. The only way to resolve it is to change the brightness of the phone, sometimes more than once or restart.
Out of curiosity, do you have adaptive brightness on or off? I've always had mine off so I wonder if this is an issue of adaptive brightness coming on momentarily or something. It only seems to happen above 1/4 or 1/3 brightness,never when it's dim. Also, tapping anywhere on the display seems to affect it.
The good news is that it's definitely not a hardware issue, which I thought it was originally.
