I've used ID for years and never encountered this problem.
I'm bringing in vector illustrations from AI (just copying and pasting them) but the strokes aren't uniform at all. I have two pictures, both with "1.5 pt" but one is way way thicker than the other.
What's going on?? I've googled this but can't really find an answer.
Thanks for reaching out. Please try the suggestions shared here https://helpx.adobe.com/indesign/using/importing-files-applications.html and let us know if this helps or if you need any further assistance.
Just to be sure this isn't caused by scaling, can you confirm that the two illustrations have the same overall dimensions for the art in Illustrator.
First of all, you should always PLACE .AI files as a rule. Copying and Pasting is not an effective workflow.
That being said, does the same thing happen if you DO "Place" them?
Your When Scaling: preferences have been set to Adjust Scaling Percentage instead of Apply to Content.
That's why you have the same stroke size appearing so differently.
To fix current art, select all and choose Reset Scale to 100% in the far upper right application menu. That will set the lines to show their true weights. You will then have to re-set the line weight to your prefered size.
Change the preference to keep it from happening in the future.