[Kevin]

I have the same problem too. The problem is occured in class-bp-members-admin.php

if ( isset( $user->user_status ) && 2 == $user->user_status ) {
    $last_active = $user->user_registered;

// Account is activated, getting user's last activity.
} else {
    $last_active = bp_get_user_last_activity( $user->ID );   //<-- ruturn empty string if not active before.
}

$datef = __( 'M j, Y @ G:i', 'buddypress' );
$date  = date_i18n( $datef, strtotime( $last_active ) );  //<--return current time if $last_active = ''

I have a fix and put it in functions.php

function filter_bp_get_user_last_activity( $activity, $user_id ) { 	
	if($activity==''){
              add_filter( 'date_i18n', function( $j ) { return ' -'; } );
	}
    return $activity; 
}; 
add_filter( 'bp_get_user_last_activity', 'filter_bp_get_user_last_activity', 10, 2 );

Hope it helps.