Applications sometimes install font files with special characters in the file name, such as using the %20 escape in the ACTUAL filename of the font file (instead of the 'space' equivalent). I have also seen %C3, %86, %2c, and %26 in font names added by applications.
When natively installed, the font installer keeps the %20 in the registry string and the font works.
Under MSIX, the font must be added to the manifest using the UAP4 reference schema, however the % character is not allowed in the relative file reference. Replacing such escapes in the reference (but leaving the captured font filename as is) causes the font to not work (as in missing without error and a different font replace for it).
I believe that the schema should be modified to accept all filenames supported natively by the OS.