Why does Alpha Arbutin turn brown over time and is it still safe and/or effective to use if it does?

I have a half-used bottle of Alpha Arbutin 2% + HA concentrated serum that I got from The Ordinary maybe about 4 or 5 years ago. And I think I remember it turned brown-ish within a year, and has gotten a bit browner since then even though I tried my best to not leave it near sunlight or other light at first. I hate letting products go to waste though and am wondering if this is safe/effective enough to use on other parts of the body that are not the face? Such as on my legs, arms, etc? I'm curious also if anyone knows and can explain the chemistry of why this serum turned brown over time while my Niacinamide 10% + Zinc 1% serum from The Ordinary bought around the same time has stayed clear? If the Alpha Arbutin has become less effective or safe over time due to its turning brown, I was wondering if someone could explain exactly what happens to the chemistry of the Aalpha Arbutin over time to cause that? I heard that certain other types of clear serums that have turned brown/expired are unsafe to use on the skin once they've changed color, and I was wondering also if anyone remembered which ones and could explain what changed in its chemistry to make those serums unsafe past its expiration date? Thanks so much to anyone that's able to answer one or more of my questions! 😊❤ I've been curious about this for a while, and enjoy learning about the chemistry of these kind of things! 🐟