We want to create a search which shows us Incidents that have Journal.Note entries with a certain category. But when we choose "Activity History" from the list it only shows us the categories from Journal.Email:
The Activity History tab on Incident is the IncidentContainsJournal relationship.
On the Journal object the Category field uses the "Category of JournalEmailCategory used in Email" picklist which only shows the Email categories and no Notes categories.
If you go to the Journal#Email.Category field it uses the same "Category of JournalEmailCategory used in Email" picklist but the Journal#Notes.Category field uses a picklist called "Category of JournalNotesCategory used in Journal.Notes".
So, the reason why this is occurring is because of the picklist that the parent Journal is using. This is the design out of the box. You can work around this by creating a new relationship to Journal#Notes so you can access those specific categories via your search as well.