Yes, I would have expected that as well, but the report disappeared.
I've now confirmed that the report is still in the database, but it cannot be loaded, trying to load the report ID directly ( /reports/### ) returns an error "The requested report could not be found." - even though the report is definitely in the database.
I've managed to reproduce this - it specifically relates to reports about profile posts made on a user profile that is subsequently merged with another user. The reports become unloadable.
Steps to reproduce:
1. create new user UserA
2. create new user UserB
3. create a profile post (from any user) on UserA's profile
4. report that profile post on UserA's profile
5. merge UserA into UserB
The report created in step 4 becomes unloadable and no longer appears in the list of reports.
The profile post that was reported still exists - it is now shown on UserB's profile.