Yep! Your display does have unevenness.
As to the cause as you correctly identified the panel its self has issues, and will need to be replaced (under warranty I hope).
But why? This gets into the chemistry of the liquid crystals and how they work. You likely have played with polarizing sheets where you take two and if you twist them 90 degrees the light will be blocked and then twist them back sigh passes. Simply put, this is a comb filter that filters light that is going in different angles of the sheet. Well it turns out this is the exact same effect LCD displays use. The crystal normally blocks the lights transmission and when power is applied to it, it becomes untwisted allowing light to pass. Here’s a good vid thats shows the effect: https://www.youtube.com/watch?v=k7xGQKpQ...
OK, so the liquid crystals them selves are like valves flexing on and off letting the light through. But, just like a water faucet it can leak! Which can happen here as well. If the crystal is held at a position for long periods of time it can loose its ability to both loosen or to tighten allowing some light to pass as well as not fully block the light. This fatigue is more common on older displays do to age. But it’s not the only cause!
Lets change things here a bit and talk about TV’s they are constantly flexing the crystals yet you don’t see this leakage within them why is that the case? Surprisingly it is! It’s simply the lack of the image from being fixed as long as we do with PC’s. This gets into the background image your Mac likely has running now with the default Sierra or High Sierra Mountain scene or what you’ve replaced it with. Which has some harsh extremes in both color and brightness throughout the scene.
So lets try this, using a plain neutral screen color so the extremes are not present. This will allow the crystals to be less stressed across the panel giving you more evenness of saturation. This won’t fix a screen thats already damaged but it will allow it to recover a bit. Also lower the brightness as well.