You can’t filter for variant attributes with the default filter. Only parents. You need a module for this, there are some available. One of them you’ll find in my signature, with this module you do not need to add attributes to variants, the module takes the filter values directly from the variants (e.g. “size”: 30, 40, 50). BUT: the module won’t work with Oxid 4.4.8. However i would strongly recommend an upgrade to the latest shop-version anyhow, because zillions of bugs have been fixed in both shop and modules since 4.4.8, and new features have been added.