Per discussion with SkySom:

Create a block requires MaterialSystem.getPartType("storage") .

One might logically expect MaterialSystem.getPartType("block") here (notwithstanding that it kinda visually conflicts with something like metal.registerPart("block"), not really a problem. )

Connected, you identified this as a concern:
var machineFrame = MaterialSystem.getPartBuilder().setName("machine_frame")
vs
var machineFrame = MaterialSystem.getPartBuilder().setName("Machine Frame")

Fixed by: 1c63310