Normal not exactly , but I know WHY it it is doing that. You installed the Medic DLC with Replace version of the DLC.. It includes meta files to replace the vanilla firetruk and Ambulance with Medic models of your choice. You need to pick one Engine from one of the packs and rename the files to firetruk and one ambulance of medics and rename those files ambulance and drop them into the vehicles.rpf in the medic4523 DLC pack. Depending on which engine and ambulance you choose, you may be required to make an additional quick change to the carvariations.meta sirensettings so that the lights work right. I can help you with that as well once you decide which vehicle you want to use.
The alternative is to open the medic4523 folder and find the vehicles.meta for the firetruk and ambulance as well as the carvariations.meta entry for firetruk and ambulance and delete them and save the metas. that will return the vanilla trucks back to vanilla state.