EntityResolverManager's job is to set the appropriate entity:<entity_type_id> types for the route parameters during route rebuilding so that they can later be upcast.
It does this by figuring out the controller method and then using reflection to see if any of the parameters is typehinted to an entity type specific interface. Thus, if you add a NodeInterface $node parameter to your controller method and have a {node} slug in your route, then you will get the actual node object, not just the ID.
This does not work for title callbacks. If - in addition to the title callback - the controller method does have a typehinted parameters, the route parameter information is updated and everything works (as this is per route). If only the title callback has a typehinted parameter, though, and not the controller method, then it doesn't work. That's what makes this a bug report vs. a feature request.
Closed: works as designed
9.5
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
There has been no response to the question asked 6 years ago nor 4 months ago. There is no indication if this is still relevant. Therefore, I am closing this issue.
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Thanks.