There is a small bug for reply message in a group. For temporary fix (upto next version release) add following code in your theme’s (child theme’s if any) functions.php

add_action( 'fep_action_message_after_send', function( $message_id, $message, $inserted_message ){
    if( $inserted_message->mgs_parent ) {
	$group = fep_get_meta( $inserted_message->mgs_parent, '_fep_group', true );
	if( $group ){
		fep_add_meta( $message_id, '_fep_group', $group, true );
	}
    }
}, 5, 3 );