A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
I can confirm this bug. The plain lines are exported to PDF as lines, so 0.5 point line will export as that same weight.
When an arrowhead is added, the line is changed to a rectangle, not a line. After opening a similar PDF in Illustrator, it appears that PowerPoint cannot or will not create a rectangle thinner than 0.0093" or 0.67 points. So any arrow thinner than that will be 0.67 points in a PDF.
Please create a slide with graphics that show the before and after as shown in your post. Then click on the small smiley-face icon in the upper-right corner of the program window and choose Tell Us What Can Be Better. Describe the issue in detail and include a screen shot showing the issue. Submitting sends the report to the Mac programming team. I've already reported this.