I only have issues with popup menus in a specific case ( cf Small bug: PopupMenu not getting closed when using trackpad (under certain circumstances) )
I never had the issue Ivan is describing, but I’m not sure I often used a comboBox in a plugin actually.
Did you try to give a parent component to your popupMenu? When having issues with popupMenu that’s often a good workaround.
see ComboBox::LookAndFeelMethods::getOptionsForComboBoxPopupMenu()
and PopupMenu::Options::withParentComponent()