Due to lack of a bug tracker I’m just posting this here
Using the TabbedButtonBar with a custom but non-global LookAndFeel makes the PopupMenu which is shown when too many tabs are open appear with the default LookAndFeel. It seems that in TabbedButtonBar::showExtraItemsMenu the line
m.setLookAndFeel (&getLookAndFeel ());
is missing, at least this is how it’s done in ComboBox::showPopup().