Engine cranking corrupts SPI driven 7segment display
Put a 100ohm@100Mhz Ferrite Bead between the Power Supply output and the input of the voltage for the digital circuit.
In the SPI line, put a 100ohm@100Mhz Ferrite Bead between the Atmega and the MAX, it can help with some noise getting coupled on your PCB.
100nF (X5R or X7R) capacitors everywhere on your PCB, for a project like this you can't go without them or your circuit can act "strange".


Is this possible to you post a print of your PCB? Maybe it needs some tweaking, it can be coupling the noise into it.

Every single electronics made for a car or everything that have a lot of noise involved needs beads and good noise protection, just an isolated power supply isn't going to work.

How to see the noise? Simple. Get a #18 copper wire, around a soda can make a loop with 3~4 turns, get it into the oscilloscope, if you moves it around the motor of the car with care you can see how much EMI it makes.