For some reason, when Castra & Voidcraft are installed, some voidcraft recipes concerning ammunition become unavailable, even though they should be completely separated.

I made a spaceship that used Voidcraft to makes rockets & uranium ammo, but after installing Castra, the recipes became unavailable for some reason.

You should get new Building called Forge that you unlock with the Castra Science. There you can craft the Void Ammo.
It was probably done for balancing reasons.

Still weird that a recipe that is 100% from another mod get restricted like that.

Also why would a new mod building be required for a recipe that could only be done on another modded building?

This definitely isn't intended. I'll look into getting this resolved soon

Just coming by to +1 this, posted in voidcraft and they said it needs to be fixed on your side.

Also +1ing this. I'm working on Voidcraft compatibility in https://mods.factorio.com/mod/voidcraft-planetary-compatibility and it is pulling recipes from the voidworks that shouldn't be available in any other type of machine. The way it's moving the recipes also prevents all the coherence- and void-flux-based versions of those recipes from being made, so you (a) can voidcraft in a machine (Forge) that should be able to, and (b) can't speed up production with void flux nor eliminate byproducts with coherence infusion.

I'm trying to figure out how best to pull the recipes back to where they belong. It'd be much easier if the recipes weren't being moved and broken in the first place.

Since this remains unresolved I went ahead and hacked in a somewhat messy fix to Voidcraft to fix the incompatibility for now. It would still be better if something like this were to be implemented.