Your car/setup which includes any replacement parts gets changed when setup's are locked which is 10 minutes before race start. You can go and check and see that the parts are changed once the lock down period has started.
Make sure you have enough Engineers and Mechanics to be able to change all the parts you want and that you have saved the setup pages.
Hi last race i did refit all on my parts and they replace it whitout the engiens part :/
But now i will only refit engiens but it removes the markte on engines
EngineE68%
when i marked it and save it , then i go to another side on the page then back and the marked one is gone