That’s a weird one. I don’t think Elevation Markers have extension lines as part of the marker other than the on screen view option which displays them to show the extent of the elevation. You’ve said that’s turned on? It isn’t something as simple as the pen for the marker range in your work environment is set to be white or even transparent if that’s possible?
Lee Hankins ArchiCAD 4.5 - Archicad 26UKI Apple Silicon 4032 macOS Ventura (13.1)