Yeah encryption isn't the issue. The only way I see to fix this is if you stop the user from switching models mid-session, or strip out the thoughts when switching models. Either way you're degrading the user experience.

If a different model is using encyrpted blocks of another model, then by definition it is no longer a session scoped bit of information. Since you can give it to any other session and another model, clearly it doesn't even have to be the same user. Therefore, there is only one (set) of universally available key(s) used by all models across all sessions.