RotRL Lamia Spell Selection Issue [SPOILER]

[osopolare]

Dear HL folks,

I'm running book 6 of the RotRL and this adventure is full of Lamias.

Some Lamias (Matriarchs/Harridans) have a special ability where they are able to learn spells from both the Sorcerer and Cleric spell lists.

When building Lamias in the editor the "Additional Spells From?" option seems to work well. I have the Lamia set up as a Sorcerer and I can also add some Cleric spells.

This breaks when I add Sorcerer levels to the creature. The casting ability is now part of class and the Cleric spells are no longer available.

I understand from another forum thread that this is a known issue. Any idea what I can do to get around it?

Thank you.