Lineweight scaling not working

I've been trying to plot a drawing from the layout, I made a viewport and scaled it 1:25. however when I preview the print (I'm printing to pdf by the way) the lineweight is out of scale and I've checked the option "scale lineweight".

Even if I uncheck it, the lineweight stays the same

The scale lineweights box does not react to the viewport scale. In fact it has nothing to do with it. It will only scale the lineweights if the plot scale has to be changed from 1:1.

For instance, lets say your existing layout is for a large format drawing like an ISO A2, and somebody wants a smaller format copy of it on A4 sized paper. You change the paper size, change What to Plot from Layout to Extents, check the Fit to Paper box (which automatically adjusts the plot scale), and then check the Scale Lineweights box. Then the lines will be scaled down by the same factor as the plot scale (not the viewport scale) was changed to fit the small paper. This of course is limited by the thinnest line your hardware is capable of printing.

The scale Lineweights box was put there as a way to prevent the lines from getting all squeezy fat and ugly when we have to give some desk jockey a small format copy of a shop drawing because he needs it in his file cabinet.

if your lines are particularly heavy on your normal layout when plotted, you will just have to adjust them thinner in modelspace.

That will be easy enough to fix if your differing lineweights are being controlled by the layer properties rather than by the object properties.