Conflicts with the Jetpack plugin
If you are using the "Jetpack WordPress plugin" maybe the "Login as User" WordPress plugin will not be working as expected.
The Jetpack disables the "send_auth_cookies" function, which allows preventing auth cookies from actually being sent to the client.
Follow the steps below to fix this issue.
1. Open the file: \wp-content\plugins\jetpack\modules\wpcom-block-editor\class-jetpack-wpcom-block-editor.php
2. Comment the line 435: // add_filter( 'send_auth_cookies', '__return_false' );
3. Save and Upload the file and the Login as User will works properly again.