Yes it's bad practice to change one side in a caliper pair.
If one side wears more than the other (to a large degree) there's a reason for it and one reason could mean braking effort has actually gone down. This is a fairly new bike, so there shouldn't be the sticking piston problems I've had on my bike. The problem can start with a brake overheat, but remains and can get worse if the pistons are sticking because all the friction effort is going through the free pistons and rotor which will run hotter.
Here are some possibilities:
Rotors not running true or badly scored.
Piston(s) sticking on the side that's not wearing.
It's quite easy to check with the rear wheel off the ground if there is too much drag. Sticky pistons is a bit more difficult, you have to suck out a little brake fluid to stop overflow, push the pistons back. If they don't go back easy they are seized. Then get somebody to slowly apply the brake, the pistons that move last are possible sticky ones.
I usually do this check with the calipers off the bike annually with an air line. A piston not moving with more than 5psi in my view is a problem.
After a short test ride using the brakes, both rotors should be about the same temperature. If both are very hot that suggests drag, but if one is very much hotter, that suggests sticking pistons. An IR thermometer is quite useful for checking.
The calipers BMW use don't have really good dust seals and boots and if the fluid isn't changed regularly you can get problems from internal corrosion.
Replacing pads won't help sticking pistons, I'd do a few checks first and if confirmed definitely go for warranty.
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