On JUCE Develop 47d441787c9af0bbf5ba0a0694fe4f516be78be9 I see a similar issue. It shows an arrow and hides the other options (even there is enough space for display). And it does not align the top of the popup menu with the top of the combobox.
Besides, it does not respect the juce::PopupMenu::Options().withPreferredPopupDirection(juce::PopupMenu::Options::PopupDirection::upwards); in another combobox (it should pop upwards to avoid overlapping the sliders below). Again, I am sure there is enough space for pop upwards:
Those issue do not exist with JUCE 8.0.8 master.
As we are going to get a point release very soon, I wish this issue could be addressed before that.