Staging not occuring in order shown

[THX1138]

If you load the save file below you'll be in control of a probe I made. If you try to activate the engines using the staging, the four stacked xenon tanks will be jettisoned instead - it doesn't matter how you arrange the stages - the xenon tanks are jettisoned before the ion engines are activated (which isn't the intended order of events).

https://www./?bx8r8k0jnq2svge