Grist, the Hunger Tide exiled with Vivien, Champion of the Wilds

I was on Arena (and I'm well aware most "bugs" on Arena are really just a player misreading an interaction, but I seek understanding) when I used [[Vivien, Champion of the Wilds]]' -2. I exiled [[Grist, the Hunger Tide]], but couldn't cast it after. I checked all other permanents and cards in graveyards (there weren't many to begin with as I had used a mana dork to cast Vivien on turn 2) and none of them had any weird interactions–this was either a bug or a rule clause that I was unaware of. Grist has a static ability that makes him a creature as long as he isn't on the battlefield, so I should have been able to cast him from exile, right? Or does it have something to do with it not being a creature on the stack or something?