If the user moves the "Characters in Reserve" window to a different location during pre-battle deployment, then any attempt to deploy any of the reserve characters (replacing someone other than Emma or Sabrina) via drag/drop fails. I've tested this out several times and every time I could swap characters freely when not moving the window, and swapping fails after I move the window.
Hm, that's a bit concerning. I'll look into this. Thanks for the report!
Annnd fixed for the next update. ;)