Cannot open PDF attachments from within Outlook when PDF Architect is set as the default app for PDF files

Windows 10 21H2 / Outlook (365 for Enterprise) / PDF Architect 8

I have an odd issue. When PDF Architect 8 is set as the default app for .pdf files, double-clicking a PDF attachment in Outlook does nothing.

I can preview the PDF, if I double click on a PDF from file explorer then it opens with PDF Architect 8.

If I change the default handler for PDF files to Adobe Acrobat Reader, then double-clicking PDF attachments from Outlook DOES open them in Acrobat Reader.

If I change the default for PDF files back to PDF Architect 8 again, then as before, double-clicking on them does nothing.

Any ideas on how I can resolve this please?