Foldable collection unexpectedly folds after adding new content

Hello, I tried to work with foldable EditorCell_Collection and have detected some unexpectable for me effect.
In one case, I created a foldable and initially collapsed EditorCell_Collection with [1…n] childeren, so, while editing, I unfolded the collection to be able create new child, added new child and the collection remained unfolded that is expected.
In another case, after I had created the same collection I surrounded the collection with another foldable and initially collapsed collection, and in this case after a child in surrounded collection had been created, the collection was unexpectedly folded (surrounding collection remained unfolded).
I had tried to setInitiallyCollapsed to false programmically after first unfolding for the reason "but what if?" yet it took no effect.
So I am confusing a bit: either it is really not proper Editor conduct or I am missing some significant point?
Thanks for any help.