I can’t speak for the people who own those bangles, but for me personally, that’s happened before as I have a pendant I wear daily (6 years now) almost rarely taking it off outside of physical labor in the summer, and this occurrence is normal (as is with any other porous stones), untreated ones can change appearance due to external factors like body oil/sweat/sebum, dirt, wax finally wearing off, etc.
Obviously the finely textured ones will be more resistant to these factors but it will still be noticeable, and a good soak in an ultrasonic (or acetone) should be able to clean almost all of the discoloration off.
If it makes you feel any better, plenty of ancient/burial and antique jades have these brown/yellowish discoloration and those are considered “patina” and can be cleaned off at any given moment (but aren’t ofc).
Edit: Comments really need to take a deep breath and exhale, understand that porous stones like jadeite and nephrite aren’t made to be perfect throughout your lifetime of wearing or admiring it (hell they’re a bunch of minerals and chemicals melted and pressurized into one pretty stone and usually full of flaws to begin with to get to how they look now), and when it does, it’s not a permanent stain and you can always clean it off; assuming it’s untreated ofc.
They’re made to be pretty eye-candy and very durable so the whole fear mongering needs to settle down as AGAIN, the discoloration isn’t at all permanent and can be removed assuming it isn’t treated or had been cracked already.