Hello,
So, we further checked this concern on your Site. The Widget that was being used on your Site for the Brands attribute wasn’t the Theme one. So, we can’t say much about its working.
We have added the WOODMART WooCommerce Layered Nav widget > and selected Brand attribute in it and applied it to All Categories: https://snipboard.io/MsmxQy.jpg
And now when we are checking the Brands filter working and it is appearing and working fine: https://gkstore.gr/product-category/gadgets/?filter_brands=oem
Please check back your Site after completely clearing the browser and server cache.
Best Regards.