When you set a MultiselectComboBox to not visible or not enabled, the drop down to select values no longer works when you then make it visible or enabled again.
Using Vaadin 14.4.7 with Version 2.5.0 of the component (this issue was there with earlier versions of Vaadin 14 as well)
A workaround I found was to wrap the MultiselectComboBox in a Span and hide the Span instead of the MultiselectComboBox.
\u0000Another work-around is to simply remove the MultiselectComboBox you want to hide and add it again (at the right position) when you want to show it again. Not ideal, but it works for me \uD83D\uDE09