Steps to repro:
1. Create some object with several parts.
2. Increase instances count.
3. Select some volume in ObjectList => all related volumes for each instance are selected in 3DScene (CORRECT)
4. Select last instance in ObjectList => all volumes (except one) of selected instance are selected in 3DScene (UNCORRECT).
ALL volumes of selected instance have to be selected in 3DScene
Fix: To avoid lost of some volumes in selection
check non-selected volumes only if 3DScene-selection mode wasn't changed
or there is no single selection in ObjectList