Hi No Ch, are you seeing this every race or did it happen just once?
Could it be that you had received the "ideal setup" daily reward? You should in that case notice in the setup screen that the "ideal setup" button has become disabled. In that case the game will always automatically apply the best setup to your car and if you change it, it will indeed revert back to the ideal setup.
Thanks for your answer. We'll close it then for now. Next time when it happens, surely check if you have the ideal setup daily reward. Also try clearing the cache from the app, if it doesn't work on the app but does work on pc. If then it still behaves wrong, you can indeed make a new report. A video of what is happening could also be useful for devs to understand what you are seeing.