Before we make any change, we'll need to validate that this indeed valid syntax according to the OAS 3.1+? Can you link me to where schema.description is defined and where its semantics are specified, so that we can be sure this is the appropriate implementation.

Thanks for the pointer to look again and sorry to have bothered you.
It turn's out that not the @Schema annotation of the generator needs to be used at a parameter — even if it have a description field — but the @Parameter annotation. Then the text moves from the „blue“ position to the „purple“ one and is rendered.

That we have sometimes two texts is our problem, and I have a way to join them if needed.

For what it's worth (as well as anyone else that sees this in the future). You can dynamically make adjustments to your spec using this library, that go into affect before render by doing something like this using the onSpecLoaded hook for the library:

recursiveFindAndUpdateAllDescriptionFields(param) {
  if (!param || typeof param !== 'object') {
    return;
  }

  if (Array.isArray(param)) {
    param.map(p => recursiveFindAndUpdateAllDescriptionFields(p));
    return;
  }

  if (param.description && param.schema.description) {
    param.descrption += `<br />${param.schema.description}`;
    return;
  }
}

onSpecLoaded(data) {
  recursiveFindAndUpdateAllDescriptionFields(data.detail);
}