Why jQuery UI sortable doesn't work correctly with flex-box container?

Question

JavaScript

In the example, to sort the blocks, you need to shake one over the other, more precisely, move the previous one from top to bottom over the next one or vice versa, the next one from bottom to top above the previous one. Otherwise sorting does not work. What could be the problem?

https://jsfiddle.net/cb6etsxg/