The situation where we noticed this, is a tree where all nodes have an icon, but not all nodes have a label, in this situation the empty labels are shown as blue squares which just doesn't feel right.
Best answer by Anne Buit
The tree is not quite prepared to only show icons and not text. What you see selected is likely the padding on the left and right side of the text, so there is no way to prevent this by shortening the title.
Feel free to create a topic in the Ideas section for support of a tree that is focused on showing icons only.