Access to report : Individual expense per month

It is not an issue : it is desiged this way.

Reports are granted with global visibility to the report : users who are granted to see the reports can see all data.
There is no restriction on the resource depending on its profile : no acces mode to data for reports (as existing for screens).
To respond to your need, you have to design your own report, something like "own expense per month".

This also applies to all reports.