info.magnolia.dam.imaging.responsive.ui.imaging.ResponsiveImageOperationChain

The class checks for file extension after the output format is created and uses the node, given by the ParameterProvider. The Node is the asset node, but the extension is located in the jcr:content node beneath the asset node.

Is it intended, that the OutputFormat formatName gets replaced regardless of the provided parameters?
You're already using
info.magnolia.dam.imaging.responsive.ui.imaging.parameters.ResponsiveImageParameter#getFormatName
which uses the substring after the last dot in the node name.