After some more investigation (the problem persisted in our code when drawing new highlights), it seems that you have to call Refresh after each change.
In the 12.2.2 code we had this and it was working fine :
....
MapHighlightOverlay.Layers.Add(Highlight)
End If
_mapview.Overlays.MoveToTop(MapHighlightOverlay)
_mapview.Refresh()
in 12.3.2 we had to add a refresh immediatly after adding the layer to the overlay, otherwise when refreshing later (after the overlay move to top), the changes in the layer would only be reflected after a zoom, and not on calling Refresh by code.
....
MapHighlightOverlay.Layers.Add(Highlight)
_mapview.Refresh()
End If
_mapview.Overlays.MoveToTop(MapHighlightOverlay)
_mapview.Refresh()
Is this by design ? Doesn’t seem very efficient.