With the addition of the changes for the AdvancedMarkers, a problem has slipped in.
The documentation of AdvancedMarkers.position says:

An AdvancedMarkerElement may be constructed without a position, but will not be displayed until its position is provided

The problem is that in every algorithm (supercluster/grid/superviewport) the MarkerUtils.getPosition function is used which assumes that position of null means lat=0, lng=0 which then leads to problems when calculating the center of a cluster.

One solution could be to return null instead of google.maps.LatLng(null) in the MarkerUtils.getPosition function and then check everywhere where the position is used if MarkerUtils.getPosition(marker) == null.