Man, we can just not win. I pluck out my grey hair sometimes as it doesn't match the rest of my hair and you have your hair the color you like with the grey and it's turning dark! Yeesh! It's frustrating, I know! And yes, this is definitely a weird thing you are describing! So, I did a little reading. Some people grey due to genetics, that will usually never revert back to an original color. Others grey due to health situations. An example is if your thyroid wasn't functioning quite properly. Vitamin B-12 deficiency, anemia or viruses can cause gray hair to appear as well. Bone loss or osteopenia can also be to blame for going grey. So, if these things turn your hair grey/white and it's not actually on your genetic time clock and the health issues are resolved, then the hair can start returning to the natural color. So that you do not feel alone, here's a whole thread of people having or knowing someone who has had this same thing happen https://www.medhelp.org/posts/Womens-Health/Gray-hair-turning-brown/show/26191 Most
of the people posting on the thread are women who went grey or white early and the darker color was welcome but you know, I certainly understand liking the color your hair is now and not wanting that! I love the look you describe you have!
I'd talk to your doctor. Think about these health things that can cause greying of hair and if there is anything different now verses then in your health. However, most of those things are issues so if they are resolved, that's a good thing, even if it makes managing your hair color harder. Then talk to your hair dresser. They can do wonders to help us have the hair we want. I have sunny blonde highlights that are not from the sun . . . ;-)