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Edit: see below

Maybe you know already, but what you can do instead is to have two Buttons On and Off, and a Led or Text indicator to show the actual status of the light.

I have done similar, but I have found that you don't need to remember "xxx_previousValue" if you immediately write the RemoteXY input OFF as soon as you see it ON, RemoteXY interface can write it OFF again later, but it will have no effect.  This creates a"one-shot" effect that works a treat.

Your code could be simplified to ....

// In setup, after RemoteXY_Init() :
strcpy( RemoteXY.textButtonLight, "ON" );

//In loop() :
if (RemoteXY.buttonLight)  // evaluates as "true" if non-zero
{
  RemoteXY.buttonLight = 0;
  lightState = !lightState;   // invert the lightstate
  strcpy( RemoteXY.textButtonLight, lightState == true ? ("OFF") : ("ON") );
  digitalWrite( PIN_LIGHT, lightState);
  }
}

It is a shame that the app doesn't emulate a "proper" touch-screen interface, so that you can do "mouse-down", "mouse-over", and "mouse-up" etc.

Since it is a simple button, with only two states "down" and "up", and there's no getting away from that. So, yes, if you want "button-up" you have to do it in code, like your suggestion.  But without the intelligence in the app, if you press a button and change your mind, you cannot just drag your finger off the button and take your finger off the screen, like you can with a mouse.