In the example provided in this defect, Light Gray Canvas, there are two separate items in the portal with the same name. One is a Living Atlas item (item ID: 979c6cc89af9449cbeb5342a439c6a76), owned by esri_livingatlas and identifiable by the Living Atlas badge. The other is a basemap item (item ID: 22f39fe2e3914fe6b1fd7e139a618a61) included with the portal and owned by esri_en.
The comparison appears to be between the esri_en item prior to an update and the esri_livingatlas item after the update, which results in a perceived change in sharing settings. In this instance, the sharing status has not changed for either item; instead, they are two distinct items with different default sharing configurations. A comparison of the item IDs in the portal confirms that these are separate items. This behavior is likely consistent across other basemap items where similar differences have been observed, as it is not reproducible when comparing the same item ID before and after an update.
During a Living Atlas update, Living Atlas items are reset to their default sharing configuration (Organization) and do not retain any manual modifications made by users, as designed. To maintain public access to web maps that include Living Atlas items, consider replacing them with non–Living Atlas basemaps (e.g., those available under the Basemaps section in the Map Viewer and owned by esri_en or other localized accounts).