In WHMCS version 8.0.4 with the six stock template Browse All is redirecting to first product group where as it should list all products in page index.php?rp=-store.
I have checked and this is the way the stock template is suppose to behave. When you navigate to Store > Browse All it will only show the first product group. Unfortunately, the functionality you're looking for here is not something that is currently offered by WHMCS stock six template.
I request all WHMCS user to vote for this enhancement.