In pixelmon 9.0.0 Goomy no matter which form you're trying to evolve it into requires it to be raining to evolve at level 40 and beyond but in the actual Pokemon games neither form of Sliggoo requires it to be raining, only Goodra does. Besides that, currently in version 9.0.0 if you have a Hisuian form pokemon like Growlithe in your party and trigger the Goomy into Sliggoo evolution Goomy will not evolve into Hisuian Sliggoo.

Steps to reproduce:
Open Pixelmon 9.0.0 under 1.16.5-forge-36.2.35
Give yourself a Goomy that can evolve on level up (anything after level 38)
Give yourself any Hisuian form pokemon and keep it in your party
Make it rain as to avoid the bug that prevents Goomy from evolving outside of rainy weather
Give your Goomy a rare candy and let it evolve into Kalosian Sliggoo

Hi there! In Pixelmon, Sliggoo needs to eveolve during a thunderstorm, not simply rain, as seen on our wiki (you might need to press the "Hisui" tab to see it's requirements).

I just tested Goomy in version 9.0.0 and evolved it during a thunderstorm and it still evolved in Kalosian Sliggoo

Gotcha, my bad! Can confirm, the gooey boy only evolves into Kalos Sliggoo for some reason! I'll pass it onto the Internal tracker to get fixed! Thanks~