Unlike ArcMap and other Esri software, when ArcGIS Pro is installed on a Windows operating system, Esri fonts (TrueType font files such as "ESRI Cartography", "ESRI Environmental & Icons", and so forth) are not installed to the C:\Windows\Fonts folder. This is because installing to this directory requires administrative privileges, and Esri must support non-administrative installations of ArcGIS Pro. Instead, ArcGIS Pro installs and uses fonts from the \Program Files\ArcGIS\Pro\Resources\Fonts folder.
However, most MXDs reference C:\Windows\Fonts for symbols, and MXDs use Windows graphic device interface (GDI) objects to load the fonts. The Windows GDI cannot load these fonts unless they are registered with the operating system. Therefore, Esri uses a Windows application programming interface (API) to temporarily register the fonts for loading, and this API works in most situations. This temporary registration is not needed if ArcMap is installed, because the fonts are installed to this directory.
Windows 10 introduced the Blocking Untrusted Fonts option, which blocks temporary font load in an enterprise through Group Policy settings. This breaks the temporary font registration setup applied by Esri's Windows API. If an MXD is imported into ArcGIS Pro with this policy enacted, and ArcMap is not installed on the machine, all symbols display in Arial type font (for example, a North Arrow may appear as the letter 'K').
As noted by Microsoft, the Blocking Untrusted Fonts setting may be enabled on the operating system for the enterprise. Having this setting enabled may result in potential reductions in functionality with applications that use memory-based fonts, such as with ArcGIS Pro.
Esri recommends not enabling this Group Policy setting, as it is not compatible with ArcGIS Pro. In addition, Microsoft no longer recommends applying this setting, as noted in a blog post from the Security Guidance team.
As an alternative, install ArcMap on the operating system or manually install the Esri fonts from \Program Files\ArcGIS\Pro\Resources\Fonts into the C:\Windows\Fonts folder.