It is not a bug,
You are downloading each and every player "Name, profile picture, Truck model, Skin model, Skin colors, every single accessory: Cabin type, wheel types of every 6 or 8, engine plate, driver plates, codriver plates, number of mirrors, sunshield type, each single light they have placed on their truck" in your radius once they are in range. Your computer can get lag to spawn truck fitting information requested depending on your Network speed, GPU Power and HDD speed, this lag might vary.
Player trucks has big number of polygons, that's why you have 1 in SP and rest are low polygon truck models as you see as AI, but in MP, all of them are big polygon calculations. Unless developers decide to show your screen every other players truck , naked meaning skinless, without accessory (like AI trucks in SP), you will get this lag. Even in SP with custom trailer & such mods, you can get this graphic lag.
You might like to disable "skin DLC's" to improve loading performance.