This happened with my daughter's AirPods. It turned out that there was quite a bit of earwax build up on the AirPod with the lower sound quality. Once the earwax was cleaned off, the AirPod sound quality returned. We went from thinking we needed to discard the product and replace it to being able to keep it in service for nearly another year and still going strong. I just cleaned them again this past week.
Here is a link to the Sony website that shows what they recommend for cleaning their products, including the WF series: [link|https://www.sony.co.uk/electronics/support/articles/00249895|https://www.sony.co.uk/electronics/support/articles/00249895|new_window=true]
Hope this helps! :)