Main and sub query - sub query not returning the desired results

I have the following SQL query used for a comments system.

SELECT
    `main`.`comment_id`,
    `main`.`comment`,
    `main`.`timestamp`,
    `main`.`replay_comment_id`,
    COUNT(`replay`.`comment_id`) AS replays
FROM `posts_comments` AS `main`
LEFT JOIN `posts_comments` AS `replay` ON `replay`.`replay_comment_id` = `main`.`comment_id`
WHERE
    `main`.`post` = "107" AND (`main`.`replay_comment_id` IS NULL OR `main`.`comment_id` in ( SELECT
            `posts_comments`.`comment_id`
        FROM
            `posts_comments`
        WHERE
            `posts_comments`.`replay_comment_id` = `main`.`comment_id` ) )
GROUP BY
    `main`.`comment_id`
ORDER BY
    `main`.`comment_id` ASC;

With the following database structure and values inside:

|------
|Column|Type|Null|Default
|------
|//**comment_id**//|int(10)|No|
|post|int(10)|No|
|author|int(10)|No|
|replay_comment_id|int(10)|Yes|NULL
|timestamp|int(10)|No|
|comment|varchar(200)|No|

== Dumping data for table posts_comments
|19|107|12|NULL|1688801931|comment 1
|20|107|12|NULL|1688801995|comment 2
|21|107|13|20|1688801995|test 1 comment for comment 2
|22|107|12|20|1688801995|test 2 comment for comment 2
|23|107|12|222|1688801995|test 1 comment for other comment

The desired result will be the returning of comments with ID 19&20 since they are the main ones and 21&22 since they are the sub comments for ID 20. Comment with ID 23 should not be returned. Now the query only returns the main comments.

In the subquery if I replace main.comment_id with 20 I get the result I want, but with main.comment_id it doesn't and I can't understand why. Any comments and ideas will be appreciated.

It wont work. 19 and 20 satisfies main.replay_comment_id IS NULL but on 21 subquery gives posts_comments.replay_comment_id = 19 which doesnt provide any result. So the child 21 and 22 are not picked.

Please check this query, if it provides you result.

			SELECT 
			  p1.`comment_id`,
			  p1.`comment`,
			  p1.`timestamp`,
			  p1.`replay_comment_id`,
			  (
			    CASE
			      WHEN p1.`replay_comment_id` IS NULL THEN 1 
			      WHEN p1.`replay_comment_id` IN (SELECT DISTINCT comment_id FROM posts_comments) THEN 1 
			      ELSE 0 
			    END
			  ) relationFlg,
			  SUM(1- ISNULL(p2.`comment_id`)) hasChild 
			FROM
			  `posts_comments` p1 
			  LEFT JOIN `posts_comments` p2 
			    ON p1.`comment_id` = p2.`replay_comment_id` 
			GROUP BY 1 
			HAVING relationFlg = 1 
			ORDER BY 1 ;