World boss icon is not showing for me at all. Has been on and off like this from Season 1.
It happens to me too. I find that icon will be missing when you open the map and the servers cannot respond quickly enough. This may be due to 1. your network ping time is less than ideal, or 2. The server is really busy.
Try keeping the map open for a little longer… I have observed one time when I’m playing on mobile network, that the icon can show up like 10-ish seconds after I open the map.
Another tip is that yellow message “World boss is about to spawn near xxx” when you login or change maps. If you know each of the world boss locations on each region, you don’t need to rely on that icon to show up.