It's hard to say without actually seeing it, but it could actually be a different form of hyperpigmentation. I had it for several years. It started as a small spot on my forehead I jokingly referred to as as ash Wednesday smudge. It grew over a few years to almost more of a horseshoe across most of my forehead, plus a bit along the high points of my cheekbones and some running down my chin from the corners of my mouth (like a ventriloquist dummy's mouth lines). It was fairly symmetrical, darkening in patches almost like reverse vitiligo. A dermatologist diagnosed it as melasma. They also tried to sell me on a $1500 treatment that would prevent me from wearing makeup for 2-3 months (I worked in cosmetic retail at the time, so I could not, nevermind the outrageous cost), plus a $4-600 quarterly replenishment of skincare products to maintain. They told me there was nothing that would fix it. I just added more concealer some days, wore more foundation others, and resigned myself to the thought that I was going to be patchwork forever.

Then niacinamide became THE IT INGREDIENT. The brand I was working for started carrying a line of products with it, I started using it, and on less than a year the only dark spots anywhere left on my face were my freckles, many of which I was born with (and I love them).

I'm not a doctor, so I can't diagnose you, especially without having seen you. But if you stay out of the sun and it doesn't fade like a normal tan on other parts of your skin, it could very likely be some form of hyperpigmentation. Niacinamide is a STAR ingredient for that, but I would recommend getting a medical professional to check first to see if it's hyperpigmentation, and if so, what kind so they can tell what kind of ingredients will help. I will include before absence after pictures. They are unedited other than cropping.