When an user goes to the project and then to the Task list and selects 'Task List' option, after clicking in 'Show all' button all tasks are displayed (no matter if there are tasks not assigned to him).
Some options to resolve this scenario could be:
1) Try to check if 'Assignments' option in Task tab is an option to resolve it
2) Try to check if using Power Filter in Task List can be an option to resolve it
3) Create a custom Portlet joining different tables like: PRTASK / PRASSIGNMENT / INV_INVESTMENTS and then include @WHERE:PARAM:USER_ID@ parameter in the WHERE clause like, using this option allow show only the assigned tasks only to logged user
AND prassignment.prresourceid = @WHERE:PARAM:USER_ID@
Using the last option can require CA Services engagement