Yes, my first thought was that it is embedded font, but the same problem also occurs in other documents, for example:
https://www.intel.com/content/dam/ww...e-usb-xhci.pdf
Document looks OK, but this font can be seen on some figures, for example on page 407, "Figure 5-25: Interrupter Register Set".
So, if there is no such font, it indeed looks like a font rendering bug.