I'm an inkscape newbie. On my desktop I have Inkscape. I draw a line and then can go to the strokewidth property and change the width.
However, I have the same program (same version) installed on my laptop and I do exactly the same and the lines are always fixed at about 2.14mm. I change the strokewidth field up and down and it doesn't affect the strokewidth.
I pulled a file from my desktop down with a line at .5mm, which showed up correctly on my laptop. So I selected the line (width 0.565mm as shown in the strokewidth) and went to Preferences and clicked "Take from selection" and it took 2.14mm rather than the selected line's 0.565mm,
In this picture you can see the thin line selected, and the thicker line I am getting, and the two different stroke widths (this is just after I clicked "Take From Selection".
This only happens on my laptop. I tried with both a pen and a simple mouse cursor and got the same problem.
After this set up, I select the pencil tool and draw another line, which I have set to red below, and as you see it didn't change the line width. I went in to the strokewidth and set it smaller and smaller until it got to zero, and the stroke width didn't change.