Problem:
A recent update has prevented the import of the default sitemap within certain solutions. When you export a managed solution with the default sitemap, four new properties have been added to the App Module Sitemap nodes in the customizations.xml. These 4 extra XML nodes/properties, currently, do not import, and cause a solution import error (there are multiple root elements).
Fix:
In the <AppModuleSitemap> tag, remove the following properties from the exported solution in the customizations.xml file.
In the next few days, a fix will be released that will prevent the four properties from being exported in the App Module Sitemap tag, and following that, a fix will be released that will allow these four properties to be exported and imported as intended.