Hi,
You're right, this is a bug on our side, and it only affects products that have variants.
Since the product page was reworked in version 6.4.0, viewing a product that has variants recorded the visit on the selected variant instead of on the main product. Listings only show main products, and your module orders them by the product_last_seen_date field, so visits made on variant products never appeared there and that list stopped updating. Products without variants kept working, which is why only part of your "recently viewed" list looked wrong.
It is now fixed: the visit is always recorded on the main product again, so ordering by product_last_seen_date fills up correctly. The fix will be in the next version (6.5.1). If you want it right away, you can download the install package of HikaShop on our end and install it on yours.