This message crops up on entries completed (some already some time ago in the past). These non-sourced residence entries are apparently generated when a census is added as source. This can be determined by comparing the person named as the last to change an entry and the date of the entry with the date at which the census was added and the person who did this. See for example Ruth Millen Dent ID: 9N41-Z6D where four residence entries have no tagged sources, or ID: 9N41-Z6W, Emily Annie Little (her daughter).

This apparently also applies to other entries (e.g. the birth registration for Ruth Millen Dent in 1853), or the burial last changed on 1 April 2021 and to other entries (1881 and 1891 census/residence entries).

I believe that I did not make the residence entries by hand nor did I notice them at the time and am now wondering whether a change in the way the underlying data is managed has caused them to appear.

Though it is not strictly necessary to "repair" the quality score since it is high already, it would certainly be easier if the entry in Quality Score contained a link to a residence entry which is without a source.