I guess it's this line in your syntax.php:
$this->Lexer->addSpecialPattern('<uml.*?>\n.*?\n</uml>', $mode, 'plugin_PlantUML');
If you would like to specify capitals in your plugin name, it must be same notation with the third param in addSpecialPattern().
-> "A valid plugin name should only contain the characters a-z and 0-9"
There's not need to use capital letters, no normal user will see the plugin name.