I’m using 0.13.2, now the issue has gone.
Moreover, I also notice a css setting which may relate to the issue.
With Obsidian 0.13.2, the aforesaid issue has gone.
So I decided to changed the examples above slightly, to test the behavior of the task list.
## Para_title_4
## Para_title_6
## Para_title_7
## Para_title_8
## Para_title_9
- This is a list with children
- a
- b
- c
- d
- e
- f
- [ ] This is a task list with children
- a
- b
- c
- d
- e
- f
## Para_title_10
## Para_title_11
## Para_title_12
## Para_title_13
## Para_title_14
## Para_title_15
## Para_title_16
## Para_title_17
## Para_title_18
## Para_title_19
## Para_title_20
## Para_title_21
## Para_title_22
## Para_title_23
## Para_title_24
With Default theme, everything is ok. I mean, the issue has gone. However, when I turned on the 3rd party theme, in my case it is "Blue Topaz”, I found the issue comes back:

Fortunately, with a wild guess, I located the cause within the .css of the 3rd party theme by accident:
ul.contains-task-list ul,
ol.contains-task-list ul,
ul.contains-task-list ol,
ol.contains-task-list ol {
position: relative;
}
Removing the ‘position: relative;’ part solves the problem. But I guess it is not best manner. So, I made a snippet:
ul.contains-task-list ul,
ol.contains-task-list ul,
ul.contains-task-list ol,
ol.contains-task-list ol {
position: initial;
}

I want to emphasize that the Obsidian 0.13.2 has solved the problem. The above css snippet is to provide a patch for the 3rd party theme, such as “Blue Topaz”, to not affect obsidian solving this problem.
Hope this piece of information helpful for those who encounter the similar problem.