I'm trying to find a way to fix stag form spell casting (some spells just won't cast in stag form) but I'm not sure how to go about it. Additionally, you really should publish your per rest abilities mod.
In my experience, it's not random at all. Some spells cast every time, others never do. One can avoid casting those spells while spiritshifted, but it's quite an undue burden on the stag considering their carnage doesn't work correctly without the community patch.