Why is my input slider not showing the actual slider in a node group? See this image:

I would like to have a slider for the &quot;Less/More&quot; value. Its range is from -1 to 1. The value itself updates, but Blender doesn't want to draw the actual blue slider.
What's the issue here?
Thanks in advance!
I figured it out. You have to create the input from a slider value in the first place. Here's an example gif:

You can't go back and change it after the fact.