There was a problem with a recent update to version 3.0. Another plugin from the wordpress.org repository has the same slug. When WC Ajax Product Filter was updated it copied that other plugin instead of the bundled one.
If the site is running the theme version 184.108.40.206 delete the current WC Ajax Product Filter. Then, go to Appearance > Install Plugins and reinstall.
Or update to the latest release 220.127.116.11, then go to Appearance > Install Plugins to update all plugins as usual.
If the widgets were changed after the wrong plugin update it’s necessary to setup the correct widgets again as indicated https://goya.everthemes.com/help-center/filters-and-variations-setup/
The correct WC Ajax Product Filter has the version number 3.9.0 and it won’t have the same problem again. It will be always updated from Appearance > Install Plugins only.
For large catalogs we recommend to use WOOF which has more features and a better performace with large datasets https://pluginus.net/support/forum/woof-woocommerce-products-filter/