The question was not over convert-ly. I'm sory if it sounds like that. It was not the intention.
Actualy I first try by re-tipeing under 2.17 and when that didn't work I went back to the original file and convert it.
I don't think it has nothing to do with convert-ly and absolutly don't think convert-ly should add the extra lines to produce (in these specific case) the previous behavior :)
Simply that, up until 2.16 the Steam.transparent only touch the steams and that is not the case in 2.17.
Obviously I didn't read all the staffs that change from version to version :)
The way Federico points me, solves the isue.
Now, What's the use case for beams without stems?
I don't think there is any case use for that. As you point out. :)
It is only a way to reproduce the png image. I know the result it's nothing like the image, but it was a solution.
The image is a crop from an explanation of interpretation of a violin studie book.
Thanks