A parallel discussion is taking place on Program-l (located at Freelists).
Based on further tests, I think these combo boxes may need to be rewritten or an NVDA specific workaround may need to be written:
A possible workaround would be to use a dedicated app module that announces new values whenever UIA element selected event is fired from the child list item object. Unfortunately, it does not resolve a major use case: NVDA will not announce current value when you move focus to these combo boxes. To mitigate this, an overlay class must be defined for the combo box itself that will return the name of the first selected child object as its value.