Andy,
The important thing to know here (and the thing that is unfortunately not obvious) is that that North Arrow is a character from a font.
If you have "embed fonts" checked, then a subset of the font characters are included with the PDF, and any user can open it and see the right symbols. This is actually on by default - but it gets unchecked by other options, or by a user who might be clicking around. This is the option you should be using.
If you use "convert marker symbols to polygons" then the fonts are converted at output time to "shapes" and that's why the user can still see the symbols. This option does get around the font problem, but polygonized fonts won't take advantage of other fancy properties of fonts such as hinting, so this should really mostly be used if you have character markers that DON'T have embeddability.
If you have _neither_ option checked, then the PDF will "reference" the fonts. If it asks Windows for a font that isn't present, then "fallback" occurs and another font is substituted, which is why you get the rotated colon instead of the north arrow - it's getting Arial or something. People typically leave both unchecked if they're trying to create a smaller PDF or if they don't want to include assets that they KNOW will be on the client machines of the people opening the PDF.
This actually isn't a pro-specific problem: same thing can happen in ArcMap too, but most of the default symbols in Esri styles are now vector markers rather than using fonts, so it happens less.
BTW, there's more info on all of these settings at https://pro.arcgis.com/en/pro-app/latest/help/sharing/overview/pdf-export.htm
tl; dr: this is expected, check the box for embed to fix it is the preferred solution.
Hope this helps! Please let me know if you have any further questions regarding this information.
Regards,
Jeremy W.