When using @Grab with a package that is already bundled inside a plugin, the version inside the plugin is used instead of the one defined in @Grab.

Example to reproduce:

1. Install Blue Ocean plugin (1.22.0) on vanilla Jenkins (2.204.2) (make sure AWS Steps plugin is not installed).
2. Create a pipeline job with the following content and disabled sandbox:

@Grab('org.yaml:snakeyaml:1.19')
import org.yaml.snakeyaml.LoaderOptions
LoaderOptions options = new LoaderOptions()
options.setAllowDuplicateKeys(false)

Result:

hudson.remoting.ProxyException: groovy.lang.MissingMethodException: No signature of method: org.yaml.snakeyaml.LoaderOptions.setAllowDuplicateKeys() is applicable for argument types: (java.lang.Boolean) values: [false]

 Without Blue Ocean plugin installed the job is able to execute the method.