To reproduce:

• Throw an invisibility potion on any vanilla LivingEntity, and the same on a geckolib LivingEntity such as the bike
• Change to spectator gamemode
• See that the vanilla living entity is translucent and the geckolib one is fully opaque

I think this is because GeoEntityRenderer doesn't have the functionality that is in LivingEntityRenderer#getRenderType (which makes sense as GeoEntityRenderer isn't exclusive to living entities, but still, it seems like an oversight).

It's actually not

Using the same rendertype as vanilla still makes it render opaque, which is very curious
I'll have to work out why that is

Nvm found it lol