I am trying to create a rainbow shape in Inkscape which I then plan to cut out on a Cricut cutter.  Here are the steps I have taken to make my shape:

1. Created a series of 8 curved lines and one straight line with the "Draw Bezier Curves and Straight Lines" tool, and set their stroke to black 5mm.
2. Selected all of the arcs & lines and selected Path > Stroke to Path.
3. Selected all of the newly created paths, selected all nodes, and then selected Path > Union.
4. Saved the project as .svg file.
5. Imported the .svg into Cricut Design Space.
6. Cricut interprets this new shape as all of the original arcs & lines PLUS the new Union path (see the "Layers" panel in the attached step6.png picture).
7. When I try to send the file to the cutter, it wants to also cut all of the arcs and lines as well as the Union path.
8. In the Cricut Design Space "Layers" panel I can hide all of the arcs and lines and it will cut just the Union path, which I what I am trying to do.

Even though I can get to the cut that I would like to make, I want to understand why all of those arcs and lines are persisting in my shape when I import them into Cricut Design Space.  When I convert Stroke to Path, does it keep the original stroke underneath the path?  Or when I choose Union, does it not merge all of the paths to one path?  If so, why are all the individual original shapes I created still there?  Is there another command in Inkscape that truly groups everything into just one single path?

Thanks for helping me understand.

Not sure why you get all the separate Layers/objects. Path1299 seems to contain all what you want. I guess you can delete all other elements; maybe select path1299 and Edit->Cut it, select all and deleted and paste in place the path1299 back onto your canvas.

Thank you!  I came to the same conclusion.  If I did select that path (which was the product of the Union), copy it, delete everything, then paste just that path, it works perfectly.  So it seems to me that actions like Stroke to Path and Union create new paths but leave the original shapes or paths behind?  That isn't how I thought it worked, but that's what seems to be happening here.

I wonder what's causing this unwanted behaviour. If that happened every time it's clearly a bug. If it's coming with an update perhaps Inkscape Preferences->System->Reset Preferences plus restart of Inkscape can cure that issue. Fingers crossed.

Will test later when back at  my studio.