It seems the ability is marked as Modal just like the fighters Defender ability, suggesting mode toggle. Its currently behaving almost like a short duration spell. However, with no listed duration, or way to know if its being affected by int (duration), or even when the ability will expire makes melee combat rather risky. Suddenly, you pop right out of shift form at the worst moment. I'm wondering if it has something to do with the fact its marked "combat only". It may be an issue with the game not being able to calculate if the character is in currently in combat correctly. Thus deactivating the ability prematurely.