Filtering “In Stock” is not displaying the in stock options

Im filtering by brand “Aero Precision”. The first page shows 5 in stock products, within that brand and a 10 pages to click through.

When I click the “In stock” checkbox, it then reduces the results to literally a total of 2 products, neither of which were included in the 5 available on the first page.

I have at least a couple dozen+ in stock products within that brand and they are being hidden. Appreciate any help

The page I need help with: [log in to see the link]