When on a lan world and a pokemom hits its evolution level , the evolve animation occurs and the pokemon evolves for a few seconds , but once its thrown out it has devolved , however its level remains the same , we have tried resseting the server blocking connections of every type except our computers and this still happens , i say this so that it is clear it is not lag , we closed every program except for minecraft , and checked our connections several times , we had to get our squirtle to lvl 20 before it finally remained evolved this has only happened with a few pokemon : magikarp , mareep , and squirtle.

I've had this problem with making evolutions "stick" too.
At first I thought it was just a problem when trading, but other players than the host seem to have it all the time.

Here's what happens:
The evolution starts and plays through like normal, but sometimes (mostly the first time a pokemon evolves) the pokemon-hotbar doesn't change the image and no evolution seems to have occurred afterwards. Redoing the evolution seems to make it work the second time around though.
After an evolution, the pokemon can revert back by either throwing it out or healing or moving it in the inventory. Relogging immediately after a successful evolution seems to save it properly so that it stays evolved afterwards.