The Modern UX Resource Manager filter on Timesheet Grid view behaves differently to the Review & Approve which contains a filter. When you add a filter then type in a name, you do not get a choice to choose a name with a check box against it, in fact, you get nothing to choose. For example where you have two or more Resource Managers with same First or last name, you will get the data for all the Resource Managers until you type in the complete name.
This is working as designed.
Release : 15.8
Component : CA PPM TIME MANAGEMENT
The Timesheet Grid View uses dynamic lookup to retrieve timesheets whereas the Review & Approve uses static lookup.