Ok well I got around to just installing it myself to check, and that's not it. It's literally just the footprint script. Delete that, and everything works fine.
As for the shiny icons, that's more of an internal Essentials bug. Vanilla Essentials sets the default shininess value for icons to 0 instead of false, so this counts as "true" when refreshing the sprites. Essentials doesn't inherently come with shiny icons, so this is never a problem under normal circumstances.
EDIT: I alerted Maruno about the shiny bug. However, if you want to fix it permanently in your own game instead of waiting for it to be inevitably added to the v21.1 Hotfixes, all you have to do is initialize @shiny = false
instead of 0 in class PokemonSpeciesIconSprite < Sprite
.
If you want to fix the Gen 8 Pack bug, just add Optional = Generation 8 Pack Scripts,1.0
to the Modular UI Scenes meta.txt file.