[Baertram]

Try function ZO_MapPanAndZoom:SetCurrentNormalizedZoomInternal(normalizedZoom)
It seems to be called from internal local functions as you click the zoom slider and maybe also if you use the mousewheel as it is called from different zoom functions there (normalized, curved etc.)

The object is stored in the local g_mapPanAndZoom which is returned by function ZO_WorldMap_GetPanAndZoom()

local mapPanAndZoom = ZO_WorldMap_GetPanAndZoom()
SecurePostHook(mapPanAndZoom , "SetCurrentNormalizedZoomInternal", function(selfMapPanAndZoom, normalizedZoom) 
d("normalizedZoom: " ..tostring(normalizedZoom))
end)

If this is not the correct one that will fire as you use the mouse wheel check any other of the update/set functions there, e.g.
:OnMouseWheel(delta)
which will call AddZoomDelta
which calls SetLockedNormalizedZoom
which calls SetTargetNormalizedZoom