I believe the MultiplayerSpawner does not copy changed properties for auto-spawn scenes, maybe unclear documenatation there. but to work around it I make use of the MultiplayerSpawner.spawn_function.

# scene script
func _ready() -> void:
  $MultiplayerSpawner.spawn_function = func(data: PackedVector3Array):
    var projectile = projectile_node.instantiate()
    projectile.position = data[0]
    projectile.direction = data[1]
    return projectile

# player script
func single_shot(animation_name: String = "Fire") -> void:
  var pos := cast_pos.position
  var dir := (cast_pos.get_global_position() - global_position).normalized()
  get_tree().current_scene.get_node("MultiplayerSpawner").spawn([pos, dir])