This occurs on taxonomy term pages which have a "_title_callback" for the route entity.taxonomy_term.canonical.
PageManagerRoutes::alterRoutes removes the route from the collection but does not create the new route with all the defaults and options of the original.
Example:
* Page name: "Taxonomy Term View"
* Results: Breadcrumbs show as "Home > Taxonomy Term View" and should be "Home > My cool term name"
Needs work
4.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
If you need tihis patch to work with #3362561, here an attempt:
Confirm that #55 🐛 Page manager does not respect existing route defaults for title callbacks Needs work successfully applies to 8.x-4.0-rc3 following resolution of #3362561.