the sorting by “Best selling” is not working for configurable products in M2.4.x
Its well working for categories with simple products.
When mixed category, first the best-selling simple products are shown, after that all the configurable products (in wrong order).
We use the plug-in from weltpixel. According weltpixel, their plugin cannot be the reason.
The whole thing was working till oct 2020 approx, it could be after the update from M2.3 to M2.4.1 ist stopped working but i am not sure..