We have a WP Plugin that dynamically generates storefront pages at a subdirectory of the merchant’s site, populated with product data the plugin syncs from our own servers–e.g., the storefront may be at example.com/shop/ with individual product pages at example.com/shop/product/123456/Item-name-here. These URLs don’t exist as freestanding pages in WP Admin; they’re generated on-demand when visited.
On some WP sites (but not all), these pages get served up with a TITLE element prepended with Page Archive – (for the main storefront page) or Page Not Found – (for individual product pages) and sometimes they include other elements (e.g. header elements) that seem to come from a Not Found template; however, the rest of these pages’ content gets generated as expected, rather than truly coming up 404.
We have been unable to reproduce this problem in our WP testing sandbox and unable to identify why this happens on the sites where it does occur, even if we install the same Theme and other Plugin(s) we can identify from their page source.
Read more here:: Plugin-generated pages use Not Found or Pages Archive templates?