The above cant be made in assemblers, is this intentional?

Nop, everything is working for me fine. There cant be a Bug, because its a nearly excact copy of the base recipe.

Battle Carriage MK1 Needs 2 x modular-armor. Cant be made in Assembler.

You can select the recipe but it will never complete.

The problem appears to be that it wants 2 modular armors and 1 rail car, but the assembler only provides two slots.

So, after the first modular armor and the first rail car goes in, the assembly machine is full (because you can't have more than one modular armor in a stack). Therefore the inserter waits patiently for a slot to open up but it never does.

I see there was a recent update to this mod, and yet the recipe for the trains hasn't been fixed so that they can be produced in assembly machines. The problem is that the assembly machine can't hold two of the armor in one slot.

Would it be so imbalanced to make the train only require one armor, or else have a prerequisite item that costs 1 armor to make?

Of note, I'm also getting a glitch when trying to make the wagons that only require one armor. The arms will grab a second armor instead of the other required part.

I changed the recipe line in data.lua from 2 to 1 modular armor. I suppose everyone who has a problem can go do this, but it would be considerate of the mod-maker to fix it so that everyone doesn't need to.
from:
cargo_wagon_create( "battle-cargo-1", 20, 1000, "grid-mk1", { r = 22, g = 187, b = 238 }, { { "cargo-wagon", 2 }, { "modular-armor", 2 } }, "armor-making-3" )

to:

cargo_wagon_create( "battle-cargo-1", 20, 1000, "grid-mk1", { r = 22, g = 187, b = 238 }, { { "cargo-wagon", 2 }, { "modular-armor", 1 } }, "armor-making-3" )

To fix my second glitch (where the arms will grab more armor instead of the lower tier wagon), I just made two requester chests, one with each item in it.