I searched the document for this rule (which is in the v0.12b pre-release), but did not find it:
Am I missing it somewhere or has it been removed?"When a Signature Spell is cast from memory (meaning not from a Magic Item or the Flexible Casting skill, and not used via another skill such as Magistarium Strike), if it hits a target but triggers a Guard, Return, or No Effect defense, the spell may be subsequently Meditated back as though it had missed."