Shiny Despawned?

I need help figuring out what just happened to me.

As far as I know, generally shiny pokemon in Pokemon ZA will not despawn. The only way they can is if you already have 10 shinies in the wild.

I have been shiny hunting a Ghastly in wild zone 3. I ended up spawning an Alpha shiny Honedge. I tried to catch it, but it was too tough. I left and went to heal and buy more pokeballs. When I returned it was still there. I tried again and failed. I then left the wild zone. My switch battery was low, so I slept it and placed it on the dock.

When I picked it back up about 10 minutes later, I was still outside the wild zone, and the shiny was gone.

I did not manually save, but I didn’t think I would need to. I will be from now on though.

What happened? How did the shiny disappear?