Have the sovereign (most likely any channeler) leading an army stack. In my case it was just one non-caster unit.
Save game.
Load game.
Click on the ARMY, not the round sovereign unit button. The "cast" option is now available.
Click on the spellbook icon. Book opens but the CAST button is not there.
This clears up when activating the UNIT to cast the spell but that's beside the point.
Reproduced that several times.
