There are always 3 Eikthyr altars, and between 3 and 5 of the other bosses.

When you click on the vegvisir it shows you the closest alter.

Not a bug, you've just found another one.

That's normal. There are multiple of every boss altar usually scattered around the world, and the stones you find will direct you to the nearest one. Eikthyr's mostly spawn near the center of the world, which can even see you getting some ridiculously close together, like this little spot (there is one on the left side behind me, and one a bit further back on the right, right next to each other):
https://steamcommunity.com/sharedfiles/filedetails/?id=2932255242

No, there are usually multiple of all altars.

there are multiple altar of every boss, in case their spawn is bad

Starting location will show you 1 possible altar but you can come across others as well when exploring. Each boss has multiple locations on the map.

Something I noticed in my first playthrough on a seed created before mistlands update... boss location tablets don't necessarily give you the closest boss altar. Had to travel quite some distance to kill Moder only to find a much closer location much later.

Spawn an Eikthyr herd. My record is 11, I don't know what the mob limit on Eikthyr is though.