The pokemon I start the game with does not evolve upon acquiring the exp needed for levelup.
I've tested it with a couple starters and it appears to affect all of them.
The bug does not occur if the pokemon reaches evolution level through a rare candy
However when the evolution level is reached through exp in combat it always results in a failed evolution:
https://gyazo.com/3dbb1dbed6abc7dafc9a89f9a99dd2d3
https://gyazo.com/fb71d3441185586d9e8c14157b2b077d

This is a known bug that we are currently in the process of trying to fix. This ticket is being merged into another ticket so all information on it can be kept in one place to be reviewed.

If this bug happens again and you can provide anymore information on any additional circumstances that made it occur (time spend online before evolving, any held items, any cancelled evolutions in the current game session before this one, etc), just post it below.

To view this new ticket, follow the following link: https://www.pixelmonmod.com/tracker.php?p=2&t=14241

In the meantime, all we can suggest to try is to leave the game, restarting your game and launcher, or restarting the server (if you play/own one) to attempt to fix it. Or default to the /evolve command upon a failed evolution until the bug is fixed.

Thank you for your report.
-Bull