The following groovy pipeline:

node {
   stage 'Do nothing'

}

node {
   stage 'Showcase'
   input """This text
should appear on a new line!"""
}

results in preserving the line break at console while removing it at the input approval box in stage view (see attached files).

To be able to present pretty formatted information to the submitter the input box should show the text the same way as the console output.