Ignition switch power interruption from run to start

A reverse biased diode, in parallel with relay coil, will freewheel coil current, resulting in slow opening of relay contacts. This is often enough to avoid interruption. Some relays have a built-in diode. It is important to observe correct wiring polarity of coil.

If you have a scope, it is possible to trigger on crank, and observe relay output in capture, with time base set to low frequency.