Thanks, that's helpful. First, for reference, the "contextual" path is the "fully qualified path relative to event's context".
In the first case, the event's context is a 'team_member', so your app has access to the information about the full path in that particular team member's account.
In the second case however, the event's context is a 'non_team_member', so your app does not have access to the information about the full path in the non team member's account. I.e., for a particular shared folder, the user can put that shared folder anywhere in their account that they want. Since your app isn't authorized to the rest of their account though, it can't see the rest of the path, e.g., any parent folders.
So, to summarize, while your app gets events for the shared folder since it's owned by your team in general, since this event is from a non-team member, it doesn't get the contextual information about that shared folder's location in that non-team member's account.
Further, there isn't necessarily a single answer as to "where" the shared folder is located in your team members' accounts. The shared folder may be at different locations in different accounts.
If you're using an app with the team member file access permission though, you can use the team member file access feature to list the members of any particular shared folder using /2/sharing/list_folder_members[/continue], and then look up the information for a particular shared folder in a particular team member's account, such as where the folder is located, using /2/files/get_metadata.