Wacom Sign Pro PDF: The font used for digital signing is illegible

The font used for digital signing in a PDF document generated by Wacom Sign Pro PDF is not displayed correctly in Acrobat Reader: the text is illegible.

I use the option to not embed the fonts into the generated PDF.

Acrobat Reader version: 2025.001.20428

Wacom Sign Pro PDF version: 4.8.0.100