Sometimes comments disappear

Sometimes I have a trouble with comments.
There you can see that this topic has 3 comments.
https://photos.google.com/photo/AF1QipMggtnV4oIEUy_9fd2CPKYQCnI36PsQwV32xf0
But there are only 2 comments.
https://photos.google.com/photo/AF1QipPhXQUWj8nZSSlucfWHNbAwz5FFSlaoXg4_BCo
In admin console I found the third comment that is not displayed in the topic
https://photos.google.com/photo/AF1QipNuIBjcVAYFnPeFxAABPvZ57lvF4iYRGzW2fDY
If I click on date of this comment I will see that situation
https://photos.google.com/photo/AF1QipMp-RM9Yxir3jMhdVAhZ-D4_xNtyT1pUkj29cc
There are 2 trees with the same comments. The red one is displayed and the green one with disappeared comment is hidden or something like this.
There is screenshot from DB. All comments look normal.
https://photos.google.com/photo/AF1QipN4irWX1usMgXhj9QdKDHvCABnOEjMxyQ6C6q8
If I change the ‘comment_parent’ of the third comment to 0, it starts to appear as a separate new comment.
https://photos.google.com/photo/AF1QipPAKMGDOnVsPIIApWNPq4ztrTwnwhwvDQ47TyY
This problem manifests itself a second time. Can someone explain the cause of this problem? And what should be done to ensure that these bugged comments are correctly displayed as a response?
P.S. Sorry for my bad english.