I have a 2020 offroad premium and it does the same thing. It's parked in a garage most of the time and still does it. My vette does it after it's washed or if it's driven in the rain. I assume it's minor surface rust on the disks. I agree, it's disappointing, but don't think there's probably a solution.

Toyota 4Runner 5th GenVehicle problems:
problem #1: Squeaky brakes on a brand new $56,000 “2020” 4runner Limited. I’ve heard it’s due to cold weather, well I live in Florida so then I was told it was due to hot weather. My reply, for a brand new automobile that costs $56,000 it should drive in Alaska and not squeak..
Problem #2: on the navigation screen a warning pops up. We’re getting warnings for “Pre-collision and vehicleVehicle Stability Control Malfunction has occurred
A malfunction in the pre-collision system has occurred. Do not rely on the system please contact your Toyota dealer immediately, your vehicle needs to be inspected. I brought it to Toyota they said they couldn’t duplicate the problem and that there’s nothing wrong with the truck.

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Hmmm...
Thats odd. I had the stealership replace the rear rotors/pads and front pads last November on my 2014. Since then I have had a squeak from my driver rear when coming up on a slow roller stop (just after it's been sitting). It was annoying, but I could live with it, but then I noticed all the dust that was stick to the wheel.
I wonder if Toyota(or their supplier) has changed the rotor material slightly and it squeaks and throws off a ton of dust. IDK, I guess it could be a bad pad too. I've had a lot of brake jobs done on vehicles over the years, and I've never had a squeaking after a week, unless the vehicle sits a long time and the rotor corrodes.
I've taken it back as I don't want to have to hand scrub the one wheel to get it decent when all the others come clean in the car wash. Nothing obvious was wrong, I'll see how it goes over the next month or two.