Why does my Bulbasaur go back a level when evolving?

[Silux555]

So weirdly enough I was testing my RomHack to discover upon evolving, my Bulbasaur went from level 15 to level 14 instead of 16.

I've made many changes that can be seen here: log

Any idea what could have happened? Maybe it's the growth-speed and exp base relation or something.

Some screens:

 

[Shiny Miner]

So weirdly enough I was testing my RomHack to discover upon evolving, my Bulbasaur went from level 15 to level 14 instead of 16.

I've made many changes that can be seen here: log

Any idea what could have happened? Maybe it's the growth-speed and exp base relation or something.

Some screens:
View attachment 166348
View attachment 166349
View attachment 166350
View attachment 166351

Maybe editing/opening in G3HS may have caused that bug

 

[RefinedPlat]

This typically happens when the growth curve of a Pokemon and its evolution are different. A Pokemon's level is essentially just a formula of how many levels their current exp would get them in their level up group. If a Pokemon evolves into a different group it will recalculate and may find itself at a very different level with the exact same exp (this never happens for obvious reasons).
You probably just need to make sure both of them are in the same exp level up group.