When new spellcaster classes are introduced, the existing scrolls seem to be updated as well and include information like the spell level requirement for the new class.
For example, the scroll "Lesser Mind Blank" has recently been changed from:
Spell Level: Wizard / Sorcerer 5
to
Spell Level: Beguiler 5; Wizard / Sorcerer 5
However, at the special properties section, it still says:
Only useable by: Sorcerer
Only useable by: Wizard
The new scrolls that are spawned after the update however, contain the following instead:
Only useable by: Beguiler
Only useable by: Sorcerer
Only useable by: Wizard
As a result, these scrolls do not stack and the old ones are probably not useable by the new classes
I have also seen this happen to Voodan and Warmage spell scrolls.
Since the existing scrolls are updated with the introduction of new classes, I suspect this might be easy to fix by adding the new classes to the "Only useable by:" in the update function where they seem to have been ommitted opposite to the "creation" function for new scrolls.