You can just teleport into the modded car that you made yourself when you're wanted which is quite annoying as cops can arrest you by just pressing F and other players can hijack the car the same way.

I got 0 idea if this has been suggested before or if it even can be fixed but hopefully, it can be done

@SAES-Developer

Well, It's a nice suggestion and fair, +1

This is intentional behaviour.

The reasoning for it has been mentioned on the forums several times in the past. Due to how MTA works with client sided vehicles in a very large amount of cases it would be impossible to enter your custom vehicle, due to it being obstructed by the other component vehicles.
The fact that cops can wrap into it as well comes from the same issue, otherwise you would not be able to be arrested while in a custom vehicle.