Don't want to write this by hand? GodCraft sets minecraft:renders_when_invisible for you with a switch.Get GodCraftOverview
minecraft:renders_when_invisible is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Keeps the entity drawn even while it is invisible, so the resource pack can decide how it should look in that state.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:renders_when_invisible": {}
}