I set up Name Preferences to display middle names.
The middle names do get displayed in the tree most of the time, but for several of my relatives they just do not want to show up in the Tree displayed names.
It's important because the 'first name, middle name' is the polite way to address older people in Russian, say, Alexey Ivanovich (Alexey, the son of Ivan).
I tried to set the displayed name explicitly with no succees.