Salesforce System Administrator Gets Logged Out When Using 'Login as User'

Disable the setting for 'Force relogin after Login-As-User'

In Salesforce Classic

1. Click on Setup.
2. In the Quick Find box, enter Security Controls
3. Click on Session Settings under Security Controls.
4. Deselect Force relogin after Login-As-User under 'Session Settings.'
5. Click on Save.

In Lightning Experience

1. Click on gear icon on the upper-right.
2. Click on Setup.
3. On the Quick Find box, enter Session Settings. Click on Session Settings under Security.
4. Under 'Session Settings' section, deselect Force relogin after Login-As-User setting.
5. Click on Save.

If the setting is enabled, an administrator must log in again to continue using Salesforce after logging out as the User. Otherwise, the administrator is returned to the original session after logging out as the User.