Shiny disappeared, and I'm incredibly bummed

Been hunting for a full form shiny Sneasel line. Meaning that, besides all regional formes, I want too collect all gender differences too.

After a total of six males, I finally found a shiny female in an outbreak. I wanted to save in front of her, but she and an other Sneasel attacked me before the menu could pop up. The other Sneasel got to me first, but when I exited the battle, the shiny was gone. I doubt she ran out of range, because she literally ran into me when the battle began. I saved there, hoping she would reload after restarting the game, but she never popped up.

What had happened here, can I retrieve the shiny (haven't left range since) and what can I do to avoid this outside of saving on time?