AB-620 - only Associate certification with prerequisites?

Yes, this looks like a documentation/credentialing inconsistency rather than a normal Associate prerequisite.

The current Microsoft Learn page for AB-620 does not list a prerequisite certification; it describes the expected experience/knowledge instead.

The certification poster apparently showing AB-620 as a prerequisite for AB-100 is different: that means AB-620 is a prerequisite for the Expert/Architect certification, not that AB-620 itself requires another certification. Microsoft’s AB-100 page lists AB-620 among the qualifying prerequisite certifications.

So I would not interpret the poster as meaning AB-620 requires a prerequisite. The Partner Center reporting issue you describe is a separate matter and would be worth raising with Microsoft Partner Center/credentials support, especially if passed exams are not generating the expected certification activity data.