Hello,
When you open the category directly like this: /kategoriia-tovarov/siropy_premium_bar/
The page performs a full load, so it renders the category layout and the description is displayed correctly.
However, when you use filtering (for example:?filter_category=siropy_premium_bar), it updates the products via AJAX without a full page reload. In that case, only the product loop is refreshed, and the category description block is not reloaded, so it won’t appear.
If you want the category page to load fully and always show the description, please use the Product Categories widget in the sidebar instead of the AJAX filter. That widget links to the real category URL.
Best Regards,