Lineweights and plotting has always been a bit tricky with AutoCAD. Then they added named plot styles and confused it a little further. A while back they added lineweight by layer which really negated the need for plot styles altogether. I have never really understood the use for named plot styles (STB) as they just name a line thickness as “thick” or “road”. But this would be apparent from a decent layer naming system as you have a layer called “road” with the line thickness set in the layers dialogue.
But sometimes someone sends you a drawing using a named plot style without the plotstyle and as its vastly quicker to set up a CTB than a STB (for me anyway) I like to convert them. To do this type CONVERTPSTLYES and then accept the the changes. This is not really an undo-able thing so save before changing!