Here’s some behavior I discovered today when I was frustrated by Inkscape. I eventually figured out what was going on. I was going to post the issue here but, I thought I would try to work it out first.

 I am using Inkscape 1.2.2 

I had a stroke created with the Pen Tool and a separate circle. I moved the circle to cover one end of the stroke. It was sort of like a lollypop.

I converted the stroke to a path and converted the circle object to a path. I selected both objects and tried using the Union on them. It wouldn't work.

I switched to the node tool. The converted stroke had six nodes. The old circle had four nodes. No problem, right? But, the Union wouldn't work. 

I changed back to the selection tool and eventually saw the message in the status bar when I selected the converted stroke. It showed that the converted stroke was a group of 2 objects. So, I ungrouped it, separated the two objects and was eventually able to complete the Union.

The reason why the Union wouldn't work was that the stroke (that I had eventually converted to a path) had a fill designated when I created/drew it. So, when it was converted to a path, two grouped objects were created.

.

Here are some things I experimented with:

1.  When you draw a single stroke with the Pen Tool (two nodes) with a stroke color indicated and no fill indicated,

then convert it to a path,

the stroke color changes to the fill color, and the stroke color is unset according to the display in the status bar.

The status bar shows that there is a path with six nodes.

.

2.  When you draw a single stroke (two nodes) with both a stroke color and fill color indicated,

then convert it to a path,

both the stroke color and fill color are unset in the status bar. But, the new path is filled with the previously set stroke color, and there is no stroke color applied.

The status bar shows that there is a Group of 2 objects on the current layer. This object cannot be joined with the Union function.

If you ungroup and separate the two objects, there is a stroke with a fill set and no stroke color set. It will be invisible. There will also be a path with a fill color only.

If you set a new stroke color and new fill color for the grouped object before ungrouping them and drag them apart, you will see that one object has the same new stroke color and same new fill color.

The second object will have the new stroke color as its fill but, no stroke color.

.

https://wiki.inkscape.org/wiki/Release_notes/1.0#Changed_behavior_of_Stroke_to_Path

Thanks, Tyler.