I also had many times problem with Fonts in PDF-Documents, generated from Affinity Designer (Version 1.6.4 / Windows-10). Even if I choose in the menu, that all Fonts should be included, it didn't work correctly.
Further more, the alternative solution to convert text in curves also didn't work correctly. The vertical lines in letters looks like bolt-type (e.g. I=I), the curves in letters displayed normal and correct.
I real like Affinity Designer, but please solve this problems soon, because if I can not be shure, that I get after hours of work a correct PDF with correct fonts, I can not use it in a proper way.