In short, if qualities are set in both the Allowed and Preferred categories, Medusa will download up to one release from the allowed category and one release from the preferred category even if a higher quality preferred release is released at a later date.
This is a proposed fix for https://github.com/pymedusa/Medusa/issues/11710 by not downloading another Allowed quality if the existing download is already an Allowed quality in order to be in line with the Wiki https://github.com/pymedusa/Medusa/wiki/Quality-Settings#custom-qualities: