Currently, as the game “changes” your spellbook every time you enter a Dungeon, it will not remember that you set your Autocast spell. So you have to set it at the start of the floor (twice if you dual-wield). That’s not too big of a problem – the issue is that if you are doing multiple floors in a row, then it removes your autocast each and every floor, even if you don’t leave the Dungeon. And even worse yet, you lose your autocast every single time you die!
Maybe let the Runescape 2007 Gold game remember our Autocast preferences for Dungeoneering permanently? This fix could also improve gameplay outside of Daemonheim, as it would mean if you swap from say the normal spellbook to Ancient Magicks, the game would remember what the last autocast you had set the last time you used Ancients, so you wouldn’t need to set it again.