I do still wonder what the actually cause is, as there's plenty on here without issue( yet) with exactly the same motor/ reduction gear, O.e magnet same oil ( not changed) so basically identical vehicles. Only variable is driving styles?
Some reports from folk had the Hyundai oil analysed and discovered high Aluminium contamination present. Again why so? I don’t think anyone (outside Hyundai) actually knows the proven failure cause for certain. There are all sorts of theories, but all seem unproven to me. Hyundai will know for certain, so why don’t they come clean and tell us.
The one connection to driving style that I suspect is relevant is that break-in wear particles are produced off gears at a higher rate if the car is hammered when new. It's been clear that the internal magnet is not doing a good job at sequestering those as it should. That's been evident simply by looking at it with a tiny video camera poked through the drain opening, which 2 of us Kona owners have done.
Those particles are circulating through precision ball-bearings over and over, and the damage caused will reduce their lifetimes ... that not being evident to the owner for many 10's of thousands of miles later when they start hearing a noise. The statistical nature of bearing lifetimes is why we see such variations of when a noise is heard. A bearing with clean oil operating within its load specifications would last some million+ miles. But when damaged the life is perhaps only 1/10 of that. The failure distribution within that 100,000 miles is down to the extent of the initial (<1,000 mile) damage and how that slowly develops into a noise as the raceway surfaces deteriorate, even with clean oil.
For anyone with a replaced or repaired gearbox, it would pay to take it easy for the first 3,000 miles, and of course add a magnetic plug and change the oil at 300 and 900 miles to clear the particles out. If the gearbox was repaired you'd be smart to remove the factory magnet and use it on your fridge door instead.
So far one Ioniq 5 owner and one with a gen-2 Kona (same as the gen-2 Niro) have carried out oil changes (as a matter of curiosity at 100,000 miles and 10,000 km respectively) and both have found black oil. All newer Hyundai-Kia EV gear reducers have integrated oil-cooled motors (as do many other current EVs) but retain one related item in common with the 64kW Soul, 28/38 Ioniq, and gen-1 Kona/eNiro, and that is a loosely-retained internal magnet. But those newer EVs don't suffer the same bearing failures.
Likely reasons for that are (a) they have a particle filter screen to protect the oil pump (for cooling the motor) which usefully sequesters particles, and (b) they have a much higher oil capacity that dilutes any contamination.
IMO, the most likely source of the blackness is simply that the loose magnet rubs aluminium off the housing under the influence of oil turbulence while driving. It only takes a tiny amount to show up. The Ioniq 5 owner had an analysis done and aluminium was through the roof. But I don't see that as a threat to the gearbox life, just evidence that something is abnormally wearing an aluminium surface.
I've fortunately never seen any black oil in my 2016 Suzuki GV manual gearbox, transfer or diffs despite hearing about some cases, but I
do pay attention to maintenance and don't abuse it. All those gear units have magnets rigidly retained in their drain plugs and despite those having the weakest magnetic strength ever known to man, do successfully retain particles. This is from the gearbox after 43,000 km. Next photo is a Kona magnet at 20,000 km, as bald as the day it was born.
I personally don't believe Hyundai-Kia understand the issue here, as it's rather obscure. If they did they would have corrected it years ago.