Pokemon gains level after evolution

So, after my Pok?mon evolved it gains another level, it evolves at level 17, and when it gets to level 17 it evolves but after the evolution it is Level 18, I'm not sure if it's supposed to be like this but I am pretty sure not, I am using Essentials v16.2

If your pokemon ends up changing level after evolution, that means they don't have the same growth rate. Level is calculated from the amount of exp the pokemon has, based of the growth rate and if it changes, the level experiences odd jumps like you've described.