Converting an issue to subtask makes "Planning Board" to be unordered

Problem

Converting an issue to subtask makes "Planning Board" to be unordered

Steps to reproduce:

1. Access "Planning Board"
2. Create a card
3. Create another card
   
4. Convert the first card to be subtask of the later card
5. Planning Board is now inconsistent
   

Workaround

This is not the expected behavior, but fortunately, there is an easy workaround.

1. Access "Tools > Configuration"
2. "General" tab
3. Click on "Optimize (repair)" in Ranking section