When connecting to a "vanilla"/bukkit server, the client is disconnected with "invalid payload REGISTER" message. Server side, the following is seen:

org.bukkit.plugin.messaging.ChannelNameTooLongException: Attempted to send a Plugin Message to a channel that was too large. The maximum length a channel may be is 32 chars (attempted 38 - 'antiqueatlas:delete_custom_global_tile

Running version 5.0.5-fabric.

Both the server and client need the mod installed. And there is no bukkit version.

Indeed. The mod should not function on bukkit, but should not break connecting to a server without it installed either.