Are you selecting Ext A off on the p/b switch? (So AVAIL shows, rather than ON)?
Or are you just telling the ground staff on the headset they are clear to disconnect the GPU when you see the engine is started and the generator is online?
It happens. Either pilots miss it or ground mechanic pulls the cable without command. But, of course, normally we disconnect with prior switching of. The same result on IFE...
Honestly I think there is no solution other than starting both engines at the gate, or live with no IFE until after second engine start.
The trick is just to see that coming and brief the cabin crew!
Looks like the only solution so far...