❓ Why does a spawner show a warning in its tooltip?

#Answer

A spawner that spawns a set mob carries block data, and the game client adds a warning line to the tooltip of any item that does. It is not an error from the plugin, and the spawner works normally once placed.

The plugin cannot remove that warning without removing the mob from the item, which would break stacking plugins that read it. On Paper 1.21.5 and newer it does hide the extra line naming the mob, so name the spawner or add the mob to its lore if players should see which one it is.