I'll go with the stupid simple first. Mainly because I have had a shop do it to me in the past.
Have you checked all your lug bolts? Did a service monkey go nuts with an air wrench and sheer one off? Check to see that they are suitably tight.
Have you inspected the tread? If there's any scalloping or cupping, you can't fix it with a balance, but one would assume that the dealer would have checked this. But it is easy for you to visually check, so don't trust them to not miss it.
How did you store the summer tires? Storing them on the treat over winter can cause a flat spot. Similarly, if it is just a soft compound it cna temporarily flat spot after sitting for hours. I had a set of tires like this on my last car. Really soft compound, on the commute for the first ~10-15 minutes it would feel like unbalanced tires until I got them warmed up enough.
What is the tire? The speed rating matters. Start pushing the speed rating and you will get some symptoms of unbalanced tires. Normally wouldn't ask for 130-140kph, but the renny can fit some tires that have a pretty low speed rating.
Anyone check the wheel for round? out of round or damaged wheels can create a situation that can't be fixed by balancing. If it's bad enough they can reduce it, but it won't eliminate it. This partial fix can show up as stable at lower speeds but vibration at higher speeds, or it can seem good at low speeds, get vibrating at higher speeds, then sort of stabilize at even higher speeds. It won't feel too bad, but it will beat up the tire faster. Quick check to do is inspect the wieghts put on the wheels to balance them. There should not be a lot. If you see a lot, something's up. If you see a lot on only one wheel, that one probably has an issue. If you have more time, Especially if one of those with more weights is on the front, try swapping it with one of your winters and taking a test drive. See if you get the vibration. Don't drive like that for extended periods though. Put it back to one set once you get an answer.
Anyone check the alignment yet? Inspect the bearings, bushing, CV joints or shocks for damage, wear, or leaks? Unlikely if the other set of tires is fine, but it should be part of the check for the complaint.
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