Fix: Wrong position when move a subAct card to a child of choice card

[QiJin-Bouvet]

Tested in TEST and QA env. When moving a subActivity card from one column to a child card of choice card in another column, the result looks wrong.
Reproduce steps:

1. In a column, make a choice card with two children
2. merge two children together, eg. card c as shown in screenshot
3. In another column, make some subActivity cards
4. Move the last subActivity card to the child card in previous column. eg. move card a to card b

Expected: card a moved to in between card b and card c
Actual: card b get two children, which is wrong.