Niantic changed some Pokemon's animation a day before you asked this question, apparently.
Niantic and Pokemon Go changed the encounter animation on certain Pokemon when you first encounter them, or by making them your Buddy and doing the “Play with Buddy” animation.
Pokemon with new Animations:
- All Kanto Pokemon
- All Galar/Alolan forms of Kanto Pokemon (except G. Weezing)
- All megas except Beedrill, Ampharos, Houndoom, Manectric, Lopunny, and Abomasnow
- Zigzagoon (regular and Galarian)
Hence, Eevee's animation was also changed on the 13/14th of August (depending upon your time zone) along with these mons.
Not only did they change animations, they also changed when the animations would happen.
When petting your Buddy in the mobile game, some Pokemon (mainly all Kanto ‘mon, regional forms, and Mega Evolutions) do a celebratory dance to show they’re happy to receive affection. An update in mid-August made it so the animation would happen when you clicked on one in the wild.
However, there is more to the story. Niantic also seemed to have removed these wild encounter animations in 2 days on 16 August.
The short-lived “happy” animations that played when you clicked on a wild Pokemon have disappeared. Niantic appears to have removed them in the latest update.
Tl;dr: Niantic introduced new animations for some Pokemon including Eevee around 14 August. They also implemented a feature the same time where these animations would also happen during wild encounters, but removed that soon in 2 days. The animations change for Pokemon remains.
Hope I helped! :)