Culprit: https://github.com/jenkinsci/git-parameter-plugin/blob/77b9ae332f6c80bac5921c5e82dc7c9695567d91/src/main/resources/net/uaznia/lukanus/hudson/plugins/gitparameter/GitParameterDefinition/index.jelly#L15
The param is not encoded and thus a "&" will break the value and create a new parameter, which will lead to the parameter being not recognized.