I have no actual answer to your problem, but I may have a logical reason for this behavior. Since you want to sort the elements of level 1 and level2, in this case all the figures mean 1st level and put things in yellow, this is level 2, when you try to put boxes in the yellow box, the limits in both axes are too small to separate this level difference. This could be because you are using the x and y axis as a reference, so when your cursor is in the middle of the x and y axis of the outer element, it is confusing. I suggest you try the option axis: "y".
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