When Affinity Publisher was introduced in the summer of 2019, I had noticed a disastrous PDF/X-4 compatibility and advised extreme caution before placing PDF pages in this layout application.
Placing six pages of the Ghent PDF Output Suite 5.0 and exporting them as PDF/X-4 now worked flawlessly:
This has become possible because Affinity has extended the Place function. Previously, when placing, an attempt was made to make the PDF page editable in the Publisher instead of simply passing the PDF objects through. In version 1.9 you can now control the behaviour of the application in the menu Layer > Resources by switching between Interpret PDF and PDF Transfer:
However, the display of the Ghent PDF Output Suite 5.0 pages on screen is still faulty.