Popup menu of ComboBox disappears immediately


I am having a strange problem with the popup menu of the combobox: When I click on the arrow the popup menu is displayed and disappears immediatly when I release the mouse button. Thus, I have to keep the button pressed to select an item. Important detail is the I am adding a lot items (4000 items) to the popup menu. If limit the items to only 100 the behaviour changes: When clicking on the arrow the popup menu is displayed and stays. You can release the mouse button and select/scroll through the item list.
I would like to have the popup menu with 4000 items like with 100 items. Any idea why behaviour changes?
Btw. I am working on macOS, I did not test on Windows and I am working on a VST plugin and have tested this with the standalone target.